Add clock_gettime related functions to macOS

Fixes https://github.com/rust-lang/libc/issues/776. The constants were retrieved on macOS 10.12.6. Tried adding `clock_nanosleep` too, but it doesn't seem like macOS' libc includes that yet.

Doesn't work on iOS.

Define DCCP constants

This PR defines constants needed to utilize [DCCP](https://en.wikipedia.org/wiki/Datagram_Congestion_Control_Protocol) in Linux and NetBSD.

[Redox] add net functions, fcntl, and close

This adds net functions and two file functions: fcntl and close.

It also adds related constants.

The net functions are in a separate file for better organization.

Bump to 0.2.31

Tweak x86 gnu images

Apparently the 17.10 image broke?

Apparently the 17.10 image broke?

fuchsia: mxio was renamed to fdio

As a side effect of the rename of the Magenta kernel to Zircon, the mxio library was renamed to fdio, and the linker directive needs to be updated.

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.