Move postmaster's RemovePgTempFiles call to a less randomly chosen place.

There is no reason to do this as early as possible in postmaster startup,
and good reason not to do it until we have completely created the
postmaster's lock file, namely that it might contribute to pg_ctl thinking
that postmaster startup has timed out.  (This would require a rather
unusual amount of time to be spent scanning temp file directories, but we
have at least one field report of it happening reproducibly.)

Back-patch to 9.1.  Before that, pg_ctl didn't wait for additional info to
be added to the lock file, so it wasn't a problem.

Note that this is not a complete fix to the slow-start issue in 9.1,
because we still had identify_system_timezone being run during postmaster
start in 9.1.  But that's at least a reasonably well-defined delay, with
an easy workaround if needed, whereas the temp-files scan is not so
predictable and cannot be avoided.