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