auth/gensec: introduce gensec_security_ops.glue in order to avoid depending on GENSEC_OID_SPNEGO being special

In future we have get more backends that can negotiate other backends,
we should keep all of them even if we require kerberos.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>