Reduce risk of accidentally running temp-install regression tests against

a mismatching installation.  Pick a default port number calculated from the
version number, and try a few times with other numbers if that one doesn't
work.  Check if we can connect to the port before starting our own postmaster,
to detect some other server already running there.  To simplify the code,
drop --temp-port option and use --port for both temp-install and pre-installed
case.