Add IPPROTO_ICMP and IPPROTO_ICMPV6 system constants

This adds the protocol value for `ICMP` in both IPv4 and IPv6 variants.

ICMP protocol constant for IPv4 defined as `1` per [RFC 792](https://tools.ietf.org/html/rfc792)
ICMP protocol constant for IPv6 defined as `58` per [RFC 2463](https://tools.ietf.org/html/rfc2463#section-1)

#### Reference
[bsd/apple: netinet/in.h](https://opensource.apple.com/source/xnu/xnu-1699.24.8/bsd/netinet/in.h)
[bsd/freebsdlike/freebsd: netinet/in.h](https://github.com/freebsd/freebsd/blob/master/sys/netinet/in.h#L42)
[bsd/freebsdlike/dragonfly: netinet/in.h](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/1f249c981c4e89e7cde1836a75b61cac36dc7ac5/sys/netinet/in.h#L64)
[bsd/netbsdlike: netinet/in.h](https://github.com/IIJ-NetBSD/netbsd-src/blob/af5d253140491f2d1816c59ecb8a4d8a8d927688/sys/netinet/in.h#L77)
[unix/haiku: netinet/in.h](https://github.com/haiku/haiku/blob/b65adbdfbc322bb7d86d74049389c688e9962f15/headers/posix/netinet/in.h#L54)
[unix/notbsd/linux: in.h](https://github.com/torvalds/linux/blob/8fa3b6f9392bf6d90cb7b908e07bd90166639f0a/include/uapi/linux/in.h#L31) - [unix/notbsd/linux: in6.h](https://github.com/torvalds/linux/blob/8fa3b6f9392bf6d90cb7b908e07bd90166639f0a/include/uapi/linux/in6.h#L134)

Adding missing WIFCONTINUED function on FreeBSD systems

This adds the WIFCONTINUED function to any FreeBSD-like system. Addresses #505

Add O_TMPFILE constant

add pipe2 to non-OSX BSD's

https://github.com/rust-lang/libc/issues/500

https://github.com/rust-lang/libc/pull/501

Run ios tests

This WIP PR runs the tests on the iOS simulator. I've tested it locally using macOS Sierra and XCode 8.2.1. I get this output:

RUNNING ALL TESTS
PASSED 6756 tests

The python script probably needs to be customized for the specific failure output from this test runner (update: newest commit should have this).

It’s not clear if these values are used or needed by the simulator. The doc only seems to mention arm values for the actual device:
https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/iPhoneOSKeys.html#//apple_ref/doc/uid/TP40009252-SW3

Pass the tests only if we find an expected positive output (rather than checking for negative output to fail the tests).

Bump to 0.2.20

Added some DragonFly-only ttycom.h constants.

I overlooked some DragonFly-only constants in the last PR.

unix: add fchdir(2)

Add missing [fchdir(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fchdir.html) function - chdir to an open file descriptor.

Fixes https://github.com/rust-lang/libc/issues/495

Fix for removed OS X constants.

https://developer.apple.com/library/content/releasenotes/General/APIDiffsMacOS10_12/Swift/Darwin.html
Fixes https://github.com/rust-lang/libc/issues/480

https://developer.apple.com/library/content/releasenotes/General/APIDiffsMacOS10_12/Swift/Darwin.html
Fixes https://github.com/rust-lang/libc/issues/480

Added defines from ttycom.h for dragonfly and freebsd.

I wasn't sure about the convention but looking at other files it seems that whatever is over 0x80000000 is c_ulong so I went with that.

Add waitid and related constants and types.

[`waitid`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/waitid.html) is a variation on `waitpid` with a marginally more convenient way of reporting the status, and a couple of handy
additional features, such as the ability to peek at an exit status without consuming it.  It's in POSIX.1-2008 and should be available on all supported Unixes.

Along with it come the type `idtype_t` and the constants `WEXITED`, `WSTOPPED`, `WCONTINUED`, and `WNOWAIT`.  The constants were already defined for unix/notbsd platforms.

The patch is currently incomplete: I'm pushing it to get CI to test platforms I don't have.  Todo list is

* [x] Add a definition of `siginfo_t` to all platforms that don't have it.
* [x] Verify that the new constants are consistent for all \*BSD platforms.
* [x] Verify that `idtype_t` is consistent across the board.
* [x] Add `link_name` annotations for `waitid` if/as necessary.

Add MSG_PEEK socket flag

#### MSG_PEEK
This PR exposes a cross-platform `MSG_PEEK` flag that a user may pass into the `flags` parameter for `recv()`, `recvfrom()`, or `recvmsg()` calls.

```
   MSG_PEEK
          This flag causes the receive operation to return data from the
          beginning of the receive queue without removing that data from the
          queue.  Thus, a subsequent receive call will return the same data.
```

In short, users may call `recv()` to "peek" at new data (or a message) without consuming it. Recall that in the normal case, calls to `recv()` consume the data from the socket's receive queue, making it unavailable to future calls.

#### Motivation
This change enables `libc` users, such as the rust standard library, to potentially add peeking functionality to TCP and UDP implementations, like `TcpStream` and `UdpSocket`, without being concerned about the highly platform-dependent nature of the flags.

(In this case, the flag's value `0x2` is very consistent, but that is not the case for many of the `MSG_*` flags. It makes sense to keep these differences confined to `libc`)

#### Reference
[bsd/apple: socket.h](https://opensource.apple.com/source/xnu/xnu-2050.7.9/bsd/sys/socket.h)
[bsd/freebsdlike/freebsd: socket.h](https://github.com/freebsd/freebsd/blob/master/sys/sys/socket.h#L418)
[bsd/freebsdlike/dragonfly: socket.h](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/1f249c981c4e89e7cde1836a75b61cac36dc7ac5/sys/sys/socket.h#L367)
[bsd/netbsdlike: socket.h](https://github.com/IIJ-NetBSD/netbsd-src/blob/af5d253140491f2d1816c59ecb8a4d8a8d927688/sys/sys/socket.h#L517)
[unix/haiku: socket.h](https://github.com/haiku/haiku/blob/b65adbdfbc322bb7d86d74049389c688e9962f15/headers/posix/sys/socket.h#L114)
[unix/notbsd/linux: socket.h](https://github.com/torvalds/linux/blob/5924bbecd0267d87c24110cbe2041b5075173a25/include/linux/socket.h#L264)
[unix/notbsd/android: socket.h](https://android.googlesource.com/platform/development/+/73a5a3baaa5089f1ab2049e5934fa5d8a3f3e76a/ndk/platforms/android-20/include/sys/socket.h#229)

It turns out that *only* FreeBSD and NetBSD proper have waitid, and that
Solaris' additional W* constants are totally different from everyone
else's, which tips me over from 'find some way to shoehorn this into
the top-level unix/mod.rs' to 'put it in the submodules, live with the
code duplication'.

Remove kind='static' from Redox linkage

Our cross compiler links binaries statically, and the rustc target has dynamic linking disabled - adding kind = "static" is not necessary.

 * OpenBSD doesn't have idtype_t or the P_* constants either
 * FreeBSD has different values for the P_* constants
 * Android gives idtype_t a different signedness
 * Disable waitid on NetBSD as it causes a link failure - I think this
   may be a problem with the test environment

Add getpeereid function

This is a wrapper around getsockopt() for getting the uid/gid of a remote Unix domain socket peer. It was added in FreeBSD 4.6 and present in all modern BSDs I checked (including Mac OS X).