Improve PostgresNode.pm's logic for detecting already-in-use ports.

Improve PostgresNode.pm's logic for detecting already-in-use ports.

Buildfarm members bowerbird and jacana have shown intermittent "could not
bind IPv4 socket" failures in the BinInstallCheck stage since mid-December,
shortly after commits 1caef31d9e550408 and 9821492ee417a591 changed the
logic for selecting which port to use in temporary installations.  One
plausible explanation is that we are randomly selecting ports that are
already in use for some non-Postgres purpose.  Although the code tried
to defend against already-in-use ports, it used pg_isready to probe
the port which is quite unhelpful: if some non-Postgres server responds
at the given address, pg_isready will generally say "no response",
leading to exactly the wrong conclusion about whether the port is free.

Instead, let's use a simple TCP connect() call to see if anything answers
without making assumptions about what it is.  Note that this means there's
no direct check for a conflicting Unix socket, but that should be okay
because there should be no other Unix sockets in use in the temporary
socket directory created for a test run.

This is only a partial solution for the TCP case, since if the port number
is in use for an outgoing connection rather than a listening socket, we'll
fail to detect that.  We could try to bind() to the proposed port as a
means of detecting that case, but that would introduce its own failure
modes, since the system might consider the address to remain reserved for
some period of time after we drop the bound socket.  Close study of the
errors returned by bowerbird and jacana suggests that what we're seeing
there may be conflicts with listening not outgoing sockets, so let's try
this and see if it improves matters.  It's certainly better than what's
there now, in any case.

Michael Paquier, adjusted by me to work on non-Windows as well as Windows