* pthread_t is defined as uint_t, so must be c_uint, not uintptr_t, just
  as pthread_key_t is already defined
* fd_set is defined as long, so must be i32/i64 based on
  target_pointer_width; this also fixes an indirect endianness issue
  encountered on sparc
* FD_SETSIZE should be defined as 65536 when target_pointer_width = 64

Fixes #515

Add AF_UNSPEC from Apple socket.h

[socket.h](https://opensource.apple.com/source/xnu/xnu-792.13.8/bsd/sys/socket.h) of Apple contains this definition so i'm not sure why it is missing from libc.
The same goes for other BSD-like OSes...
Is there some particular reasons why it is so?

Add types for `aarch64-unknown-freebsd`

Add wcstombs

Add `wcstombs` function to convert from wide chars to multibyte chars.

The `wcstombs(`) function conforms to ISO/IEC 9899:1999 (ISO C99)

Fix the i686-unknown-linux-musl target CI

`aarch64.rs` was copied and adapted from `x86_64.rs`.

The wcstombs() function conforms to ISO/IEC 9899:1999 (``ISO C99'')

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.