s4:process_prefork: create new messaging context for the master process

s4:process_prefork: create new messaging context for the master process

In order to allow the before_loop() hook to register messages or event
handlers, we need to fix up task->event_ctx and create a new
task->msg_ctx. It also means the struct task_server pointer
changes in the master before_loop() hook.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

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