Use the same code we already applied in duplicate_oids and unused_oids
to let this script find Catalog.pm without help.  This removes the need
to supply a -I switch in most cases.

Also, mark the script executable, again to follow the precedent of
duplicate_oids and unused_oids.  Now you can just do
"./reformat_dat_file.pl pg_proc.dat"
if you want to reformat only one or a few .dat files rather than all.

It'd be possible to remove the -I switches in the Makefile's convenience
targets, but I chose to leave them: they don't hurt anything, and it's
possible that in weird VPATH situations they might be of value.