Fix duplicate_oids and unused_oids so user needn't cd to catalog dir.

Fix duplicate_oids and unused_oids so user needn't cd to catalog dir.

Previously, you had to cd into src/include/catalog before running either
of these scripts.  That's a bit tedious, so let's make the scripts do it
for you.

In passing, improve the initial comments in both scripts.  Also remove
unused_oids' code to complain about duplicate oids.  That was added in
yesterday's commit 5602265f7, but on second thought we shouldn't be
randomly redefining the script's behavior that way.

John Naylor and Tom Lane

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