Enusre that `no_core` is turned on, and while here update the `no_std`
header to be unconditionally applied.

Closes #1267

This reverts commit ae2663a7.

Moved inotify_rm_watch out of the generic notbsd target and into specific linux and android targets because the second argument has a different type in android and linux.

This commit adds definitions for setgrent, endgrent, and getgrent
which were missing (but available for FreeBSD and other *nix systems).

A follow-up from #1252.

CC @asomers.

r? @gnzlbg.

and Solaris-derived distributions (i.e. illumos).  In addition, a number
of missing definitions (and compatability functions) that have been
found necessary to run a number of rust binaries on illumos have been
added.

Portions were contributed by Mike Zeller <mike@mikezeller.net>