LDAP: Only run the POSIX check with a GC connection
    
    Previously, we used to run the POSIX check also with an LDAP connection.
    This was wasteful, but worked, so the waste wasn't the biggest problem
    -- the approach would only cause problems with the following patch which
    uses a NULL search base to search the Global Catalog, because searching
    with a SUBTREE scope and a NULL base returns a referral with an LDAP
    connection.
    
    Instead, this patch uses a heuristics (whether the connection ignores
    the offline state) to check if the connection is a POSIX one and if it
    is NOT, then skips the POSIX check.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>