De-coupled the mapping of a Windows to UNIX username from the Get_Pwnam

De-coupled the mapping of a Windows to UNIX username from the Get_Pwnam
username case conversion wrapper. It is now (very) explicit where we are
mapping between an incoming Windows username, and when we are doing a
UNIX password entry lookup, which may change the case of the given
username.
This makes things *much* clearer (IMHO:-) and will ease the adding of
the 'groupname map' parameter, and the addition of the special 'jeremy'
mode for Samba where unix users will not be needed. (We must think of
a better name for it :-).
Jeremy.
(This used to be commit fb6ed81844e7cb6049749e43ac9b4adfaf4ca2de)