Don't just assume that the next port is free; it might not be, or
if we're really unlucky it might even be out of the TCP range.
Do it honestly with two get_free_port() calls instead.

This is surely a pretty low-probability problem, but I think it
explains a buildfarm failure seen today, so let's fix it.

Back-patch to v11 where this script was added.

Discussion: https://postgr.es/m/25124.1568052346@sss.pgh.pa.us