unused_oids was previously a shell script, which of course didn't work at
all on Windows.  Also, commit 372728b0 introduced some other portability
problems, as complained of by Stas Kelvich.  We can improve matters by
converting it to Perl.

While we're at it, let's future-proof both this script and duplicate_oids
to use Catalog.pm rather than having a bunch of ad-hoc logic for parsing
catalog headers and .dat files.  These scripts are thereby a bit slower,
which doesn't seem like a problem for typical manual use.  It is a little
annoying for buildfarm purposes, but we should be able to fix that case
by having genbki.pl make the check instead of parsing the headers twice.
(That's not done in this commit, though.)

Stas Kelvich, adjusted a bit by me

Discussion: https://postgr.es/m/37D774E4-FE1F-437E-B3D2-593F314B7505@postgrespro.ru