Add uid_from_user() and gid_from_group(), derived from pax's cache.c.
It replaces the existing pwcache.c functions user_from_uid(3) and
group_from_gid(3) with the pax equivalents. Adapted from NetBSD
(mycroft) changes from our own pax's cache.c. OK guenther@

Use reentrant versions of getpw{nam,uid} and getgr{nam,gid} within
libc to avoid reusing the static buffers returned by the non-reentrant
versions. Since this is inside libc we can use constants for the
buffer sizes instead of having to call sysconf().
OK guenther@ deraadt@

Wrap <grp.h> so that calls go direct and the symbols are all weak.

Use reentrant versions of getpw{nam,uid} and getgr{nam,gid} within
libc to avoid reusing the static buffers returned by the non-reentrant
versions. Since this is inside libc we can use constants for the
buffer sizes instead of having to call sysconf().
OK guenther@ deraadt@

Wrap <grp.h> so that calls go direct and the symbols are all weak.