s4: server: Add a tevent signal handler for SIGTERM.

s4: server: Add a tevent signal handler for SIGTERM.

Simplify by removing global state we don't need now
we're called by tevent (and in the short window where
we're installed by CatchSignal but before we install
the tevent handler we don't need the complex global
state handling as we have no forked children).

We now have access to struct server_state on all
exit paths - next commits will stop using talloc autofree context.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>