Add accept4() for NetBSD, Illumos and Solaris

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