Define additional IPPROTO_* constants.

Define additional IPPROTO_* constants, e.g. for DCCP and SCTP. These constants are standard across all system types -- the values are those assigned to the protocol by the IANA:
https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml

Bring back cfg_target_vendor, which is still used

r? @alexcrichton https://github.com/rust-lang/rust/pull/44515#issuecomment-330036756

Update s3 download locations

Add additional interface flags (IFF_)

Adds the three missing IFF_ constants (IFF_LOWER_UP, IFF_DORMANT, and IFF_ECHO) per nix-rust/nix#764

Make stdbuild a cargo feature

Also invert the default feature / no std arrangement. The default
still uses std, but no_std now requires opt-in instead of opt-out.

This is work toward removing the shim in rustc.

Also remove unneeded platform gating

Add timerfd API on Linux

This change adds the Linux-specific timerfd API to libc.

```C
#include <sys/timerfd.h>

int timerfd_create(int clockid, int flags);
int timerfd_gettime(int fd, struct itimerspec *curr_value);
int timerfd_settime(int fd, int flags,
                    const struct itimerspec *new_value,
                    struct itimerspec *old_value);
```

The timerfd API has been available since kernel 2.6.25 and in glibc since version 2.8.

I'm not sure if I put the changes in the right places, so please review with great care.

Linux/Android: re-add initgroups

This was removed in #742

This change adds the Linux-specific timerfd API to libc.

Add networking symbols for uclibc/x86_64

Add Solaris 10 support

Haiku updates

Remove caching and fix FreeBSD tests

Unlike Illumos and Solaris 11, Solaris 10 does not support posix_memalign, so this change is needed for Solaris 10 support.

apple: Add VM_* constants from mach/vm_statistics.h

Closes #736.

Fix compile on sparc64

Closes #736

Fix dev_t minor() bitmasking on Linux

This code appears to be modeled on the macros in glibc bits/sysmacros.h
(since Glibc 2.26). Fix the masking of bits for minor() to match that
implementation, which also corresponds with the explanatory comment in
that file.

This code appears to be modeled on the macros in glibc bits/sysmacros.h
(since Glibc 2.26). Fix the masking of bits for minor() to match that
implementation, which also corresponds with the explanatory comment in
that file.