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

define INADDR_* constants in unix platforms

PTHREAD_PROCESS_PRIVATE needs to be a 16-bit type on Solaris

Compiles of libc on Solaris fail because we're trying to assign a 32-bit constant to a 16-bit struct member.

Bump version to 0.2.28

This contains PR #686.

unix: add fexecve()

Add missing fexecve() - execute program specified via file descriptor.

Reference: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html

Add offsets in user_regs_struct from sys/reg.h

Add missing fexecve() - execute program specified via file descriptor.

Reference: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html

Update crate version

Disable revocation checking temporarily on AppVeyor

Add signalfd and related to android

Add missing flags to Linux and Android

Add all current fallocate flags

Also move them to the common Linux module, as they are not platform-specific.
See include/uapi/linux/falloc.h.

Add mac structs

With this one, I'll have almost all the structs I need directly in libc. :)

Restore SYS_* constants to the c_long type

cc https://github.com/rust-lang/libc/pull/672#issuecomment-316406664

glibc support is behind, as indicated by
https://github.com/karelzak/util-linux/blob/master/sys-utils/fallocate.c

Also move them to the common Linux module, as they are not platform-specific.
See include/uapi/linux/falloc.h.

Add syscall table for x86 and x86_64

I'm wondering if we should use `SYS_write` or `__NR_write`. Both are available to C programs.