auth/gensec: enforce that all DCERPC contexts support SIGN_PKT_HEADER

auth/gensec: enforce that all DCERPC contexts support SIGN_PKT_HEADER

That's currently always the case and will simplifies the callers.

WORKS now???
TDB_NO_FSYNC=1 buildnice make -j test FAIL_IMMEDIATELY=1 SOCKET_WRAPPER_KEEP_PCAP=1 TESTS='samba4.rpc.lsa.secrets.*ncacn_np.*Kerberos.*Samba3.*fl2000dc'
and
TDB_NO_FSYNC=1 buildnice make -j test FAIL_IMMEDIATELY=1 SOCKET_WRAPPER_KEEP_PCAP=1 TESTS='samba3.rpc.lsa.*ncacn_ip_tcp.*nt4_dc'

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sun Dec 23 21:33:51 CET 2018 on sn-devel-144