dcesrv_core: Add dcesrv_loop_next_packet()

dcesrv_core: Add dcesrv_loop_next_packet()

This is used by the helpers of samba-dcerpcd: When accepting a DCERPC
client, normally the server engine would read the initial bind
packet. In case of samba-dcerpcd the bind packet will already be read
from the socket, so we need to inject it into the rpc server engine
externally.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>