pg_ctl idempotent option

Here is a patch to add an option -I/--idempotent to pg_ctl, the result
of which is that pg_ctl doesn't error on start or stop if the server is
already running or already stopped.

The previous discussion was at
<http://www.postgresql.org/message-id/1253165415.18853.32.camel@vanquo.pezone.net>.