postgresql: Fix shutdown

Postgres was taking a long time to shutdown.  This is because we were
sending SIGINT to all processes, apparently confusing the autovacuum
launcher.  Instead it should only be sent to the main process (which
takes care of shutting down the others).

The downside is that systemd will also send the final SIGKILL only to
the main process, so other processes in the cgroup may be left behind.
There should be an option for this...