s4: process_standard: Add a simplified SIGTERM handler based on code from source4...

s4: process_standard: Add a simplified SIGTERM handler based on code from source4/smbd/server.c. Use from a tevent handler added to standard_accept_connection() and standard_new_task()

Allows us to be independent of parent SIGTERM signal handling.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Sat Apr  8 16:21:57 CEST 2017 on sn-devel-144