s4-rpc_server:getncchanges Add "old Samba" mode regarding GET_ANC/GET_TGT

s4-rpc_server:getncchanges Add "old Samba" mode regarding GET_ANC/GET_TGT

This emulates older verions of Samba that fail to implement
DRSUAPI_DRS_GET_ANC correctly and totally fails to support
DRSUAPI_DRS_GET_TGT.

This will allow testing of a client-side fallback, allowing migration
from sites that run very old Samba versions over DRSUAPI (currently
the only option is to attempt an in-place upgrade).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15189

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