s4-auth: rework map_user_info() to use cracknames

s4-auth: rework map_user_info() to use cracknames

to properly support multi-domain forests we need to determine if an
incoming username is part of a known forest domain or not. To do this
for all possible SPN forms, we need to use CrackNames.

This changes map_user_info() to use CrackNames if a SAM context is
available, and asks the CrackNames services to parse the incoming
username and domain into a NT4 form, which can then be used in the
SAM.

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>