Add a new header file for functions in lib/util/util.c.

This allows public headers to not include samba_util.h, but rather
specific header files under lib/util.

Signed-Off-By: Jelmer Vernooij <jelmer@samba.org>
Reviewed-By: Andrew Bartlett <abartlet@samba.org>
Reviewed-By: Stefan Metzmacher <metze@samba.org>
