s4-process_single: Use pid,task_id as cluster_id in process_single just like process_...

s4-process_single: Use pid,task_id as cluster_id in process_single just like process_prefork

This avoids two different process single task servers (eg the drepl
server) sharing the same server id.  The task id starts at 2^31 to
avoid collision with the fd based scheme for connections.

Fix-bug: https://bugzilla.samba.org/show_bug.cgi?id=9598

Reported-by: Matthieu Patou <mat@matws.net>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Sat Jan 26 16:13:05 CET 2013 on sn-devel-104