Add KERN_USERMOUNT and KERN_ARND to don't check list on OpenBSD

These [two constants are no longer in OpenBSD](https://github.com/openbsd/src/blob/3f376b0849a3d7e59ed23ac6dba2b729fc2c7589/sys/sys/sysctl.h#L133-L140). They were preventing the the tests from passing. With this change the test now pass:

```
CC=egcc cargo run
   Compiling libc v0.2.29 (file:///home/vagrant/libc)
   Compiling libc-test v0.1.0 (file:///home/vagrant/libc/libc-test)
    Finished debug [unoptimized + debuginfo] target(s) in 11.7 secs
     Running `/home/vagrant/libc/target/debug/libc-test`
RUNNING ALL TESTS
PASSED 5935 tests
```

They are still present in bitrig so I moved them there.

A few missing *BSD poll flags

Added a few missing BSD poll flags for completeness.

Port #672 and #686 to Android.

I'd rather avoid the duplication, but this is discussed in #707.

Add ptrace constants.

It has been added to glibc in version 2.26.

For some reason, they are in the NDK sources but
they are not available in the tests. I don't know what
Android does here, but nothing that makes sense.

Add setresgid & setresuid to FreeBSD and OpenBSD

These functions are both present in [FreeBSD](https://www.freebsd.org/cgi/man.cgi?query=setresuid&apropos=0&sektion=0&manpath=FreeBSD+11.1-RELEASE+and+Ports&arch=default&format=html) and [OpenBSD](http://man.openbsd.org/setresuid).

Port #672 and #686 to musl.

I'd rather avoid the duplication, this is discussed in #707.

They have been added in musl 1.16 but updating musl is non-trivial.

Bump to version 0.2.29

Rationale for release: My changes in https://github.com/nix-rust/nix/pull/688 are blocked on getting a libc release out with the missing constant  present.

Add IPV6_JOIN_GROUP and IPV6_LEAVE_GROUP to uclibc

Hello! Just some minor fixes so that net2 compiles correctly on uclibc

Add EVFILT_EXCEPT constant

add SO_RCVBUFFORCE for s390x

Add missing SO_TIMESTAMP constant for OpenBSD like OSes

Added to mod.rs as both [OpenBSD](https://github.com/openbsd/src/blob/e35050970e3e2b822e383a5e844e4c4402eebccc/sys/sys/socket.h#L97) and [Bitrig](https://github.com/bitrig/bitrig/blob/a15267d8249fe8d329eb34048605c59a394a02ac/sys/sys/socket.h#L100) have it.

Required for https://github.com/nix-rust/nix/pull/688

change type of INADDR constants to in_addr_t

These should be `in_addr_t` (u32) instead of `c_int`

This is a breaking change but it is just breaking newlib platforms for now.

@alexcrichton Do you prefer that I split the definition in submodules, to keep `c_int` in newlib and update the others ?

Add CI badges