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@

Add _shadow variants to the two popular getpw functions (uid and nam).
This version of the function will always open the secure/shadow/master
password files. Soon, the regular variants of these functions will not.
(Intermixing shadow and regular gets a little weird; don't do that.)
Not using struct spwd and getspwnam functions to reduce churn in callers.
Should just be a one line diff in most places.
ok deraadt

Wrap <pwd.h> so that calls go direct and the symbols are all weak.
Hide bcrypt_autorounds(), prefixing with an underbar for static builds.