s4:messaging: make sure only imessaging_client_init() can be used with a wrapper...

s4:messaging: make sure only imessaging_client_init() can be used with a wrapper tevent_context wrapper

imessaging_client_init() can be used with a wrapper tevent_context,
but only if a global messaging_dgm_ref() already exist.

All other uses of imessaging_init() and imessaging_client_init()
require a raw tevent_context.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Thu Jul 12 02:23:37 CEST 2018 on sn-devel-144