winbindd: restore SEC_CHAN_NULL fallback in cm_connect_netlogon_transport

winbindd: restore SEC_CHAN_NULL fallback in cm_connect_netlogon_transport

This partially reverts commit d7e31d9f4d9ce7395e458ac341dd83ac06255a20
"winbindd: Use rpccli_connect_netlogon" and restores handling of SEC_CHAN_NULL.

Without this we fail to enumerate domains in trusted forests so users
from any child-domain (or tree-root) in the trusted forest can't login
via eg SMB.

This is a temporary hack that will go away once we get rid of the trusted domain
list.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13167

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>