dsdb: Check for userPassword support after loading the databases

dsdb: Check for userPassword support after loading the databases

The net result of this is only that userPassword values (which were
world readable when set) would still be visible after userPassword
started setting the main DB password.

In AD, those values become hidden once the dSHeuristics bit is set,
but Samba lost that when fixing a performance issue with
f26a2845bd42e580ddeaf0eecc9b46b823a0c6bc

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13378

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>