auth/gensec: make sure we keep a DCERPC_AUTH_TYPE_SCHANNEL backend if required

auth/gensec: make sure we keep a DCERPC_AUTH_TYPE_SCHANNEL backend if required

Even with CRED_MUST_USE_KERBEROS we should keep the DCERPC_AUTH_TYPE_SCHANNEL
backend arround, this can only be specified explicitely by the caller
and cli_credentials_get_netlogon_creds() != NULL is the strong indication
that the caller is using DCERPC_AUTH_TYPE_SCHANNEL *now*.

With trusts against AD domain we can reliable use kerberos and netlogon
secure channel for authentication.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>