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 definition of ucontext_t for macOS

Remove MSVC's wmemchr declaration

Bump version to 0.2.68

Add constants from `linux/in.h`

Add reenterant shadow.h functions

Enable `s390x-unknown-linux-gnu` on CI

Tweak `MAP_SYNC`

Add missing AT_ constants

Add AT_EACCESS to Linux, Solaris, and Fuchsia.

Add AT_SYMLINK_FOLLOW, AT_REMOVEDIR, and  _AT_TRIGGER to Solaris.

Unify mmap flags

add macOS mount flags

https://github.com/apple/darwin-xnu/blob/master/bsd/sys/mount.h#L288

Reduce checks for demoted apple 32-bit targets

Add F_OFD_* constants

Update badges metadata

Add constants from linux/keyctl.h

Move linux/reboot.h constants into linux module

Solarish: missing declaration of port_send and port_sendn

Add `CLOCK_TAI` to linux_like/mod.rs

Add MINSIGSTKSZ and SIGSTKSZ definitions for MacOS

Add IP6T_SO_* options definition

Add wait4 for macOS and FreeBSD