Remove more uses of "extern struct current_user current_user;".

Remove more uses of "extern struct current_user current_user;".

Use accessor functions to get to this value. Tidies up much of
the user context code. Volker, please look at the changes in smbd/uid.c
to familiarize yourself with these changes as I think they make the
logic in there cleaner.

Cause smbd/posix_acls.c code to look at current user context, not
stored context on the conn struct - allows correct use of these
function calls under a become_root()/unbecome_root() pair.

Jeremy.