Optimize member/memberof searches in LDAP
When investigating if member/memberof attribute is direct/indirect
we do a lot of LDAP SCOPE_SUBTREE searches when we actually search
just for one item. Make sure we search only with SCOPE_BASE to improve
the performance.
One not so efficient iteration was also changed to list comprehension
to speed things up a tiny bit.
https://fedorahosted.org/freeipa/ticket/1885