The pr #1870 introduced safe_f! macro, which made some functions like
WIFEXITED and WEXITSTATUS const and safe on linux_like platform only,
which causes inconsistency when trying to use those functions in crates
compiled across multiple platforms, as using unsafe on those functions
will generate unused_unsafe warning on linux platforms and lack of
unsafe block will fail compilation on non-linux platforms.

To avoid the inconsistency, this commit applies the same macro for all
the same functions on other platforms too.

At time of writing, illumos systems do not provide an implementation of
the openpty() and forkpty() wrappers provided on some other UNIX
systems.  While we expect to grow an implementation, it seems prudent to
provide a compatibility routine here first to unblock illumos support in
the popular nix crate.

This series of routines allows the caller to determine the credentials
of another process by pid, or of the process on the remote end of a UNIX
domain socket.  The ucred_t is an opaque object with accessor routines,
and must be freed through ucred_free(3C) after use.

Initially the EPOLLEXCLUSIVE definition was hidden on the illumos
platform as it lacked explicit support.  After further review, it was
concluded that EPOLLEXCLUSIVE can safely be considered a no-op, when not
fully implemented by the OS, making it safe for use on illumos.

SmartOS is a downstream distribution of illumos, and carries a number of
additional patches that are not commonly available in other illumos
distributions.  They are also not found in Oracle Solaris.  With these
removed, the libc tests pass on a stock illumos system running
OpenIndiana.

This change fixes two issues.  First, the current cfmakeraw()
implementation in this crate appears to be making a stack copy of the
input "struct termios" before modifying it, rather than correctly
modifying the original through the pointer.  Before this modification
the routine did not, thus, set the flags for raw mode.

Second, we address the default settings of the MIN and TIME terminal
options.  On at least FreeBSD and Linux systems, the modern default
value for MIN appears to be 1; i.e., block and wait for at least one
input byte.  On most Solaris and illumos systems, the MIN control
character slot overlaps with EOF, and thus has a default value of 4.
This breaks at least the examples in the "termion" crate, and probably
quite a lot of other software written first and foremost for Linux
systems.  We need to force the MIN value to 1 while switching to raw
mode.

References:
* NetBSD (became available with 8.0):
  http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/sys/socket.h?annotate=1.129&only_with_tag=MAIN
  http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/sys/accept4.c?annotate=1.2&only_with_tag=MAIN
* Illumos:
  https://illumos.org/man/3socket/accept
  https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/sys/socket.h
  https://github.com/illumos/illumos-gate/blob/master/usr/src/lib/libsocket/socket/weaks.c
* Solaris:
  https://docs.oracle.com/cd/E88353_01/html/E37843/accept-3c.html

Add AT_EACCESS to Linux, Solaris, and Fuchsia.

Add AT_SYMLINK_FOLLOW, AT_REMOVEDIR, and  _AT_TRIGGER to Solaris.

This triggers a warning on a recent nightly, which in turn breaks CI due to
`#![deny(warnings)]` in libc-test/build.rs

This is needed because Firefox now uses slice-deque rust crate.

It's *msg_prio* in both manpages and posix.