This changes the behavior of the start and stop actions to exit
successfully if the server was already started or stopped.

This changes the default behavior of the start action:  Before, if the
server was already running, it would print a message and succeed.  Now,
that situation will result in an error.  When running in idempotent
mode, no message is printed and pg_ctl exits successfully.

It was considered to just make the idempotent behavior the default and
only option, but pg_upgrade needs the old behavior.