occasional startup failures

Every so often buildfarm animals (nightjar and raven recently, for
example) report failures on starting up the postmaster. It appears that
these failures are due to the postmaster not creating the pid file
within 5 seconds, and so the logic in commit
0bae3bc9be4a025df089f0a0c2f547fa538a97bc kicks in. Unfortunately, when
this happens the postmaster has in fact sometimes started up, and the
end result is that subsequent buildfarm runs will fail when they detect
that there is already a postmaster listening on the port, and without
manual intervention to kill the "rogue" postmaster this continues endlessly.

I can probably add some logic to the buildfarm script to try to detect
this condition and kill an errant postmaster so subsequent runs don't
get affected, but that seems to be avoiding a problem rather than fixing
it. I'm not sure what we can do to improve it otherwise, though.

Thoughts?

cheers

andrew