nbt_server: Factor out packet generation for netlogon reply

nbt_server: Factor out packet generation for netlogon reply

This also fixes an inaccuracy (or even a bug?):
The previous code pulled the reply mailslot always through
req.logon.mailslot_name, which is the union for
LOGON_SAM_LOGON_REQUESTs. The LOGON_PRIMARY_QUERY must be referenced by
req.pdc.mailslot_name. It might have worked by chance, but this should
be more correct.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Tue Feb 27 14:10:14 CET 2018 on sn-devel-144