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 5602265f, 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

src/include/catalog/duplicate_oids

 #!/usr/bin/perl
-
-use lib '../../backend/catalog/';
-use Catalog;
+#----------------------------------------------------------------------
+#
+# duplicate_oids
+#    Identifies any manually-assigned OIDs that are used multiple times
+#    in the Postgres catalog data.
+#
+#    While duplicate OIDs would only cause a failure if they appear in
+#    the same catalog, our project policy is that manually assigned OIDs
+#    should be globally unique, to avoid confusion.
+#
+# Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
+# Portions Copyright (c) 1994, Regents of the University of California
+#
+# src/include/catalog/duplicate_oids
+#
+#----------------------------------------------------------------------
 
 use strict;
 use warnings;
 
+# Must run in src/include/catalog
+use FindBin;
+chdir $FindBin::RealBin or die "could not cd to $FindBin::RealBin: $!\n";
+
+use lib "$FindBin::RealBin/../../backend/catalog/";
+use Catalog;
+
 my @input_files = (glob("pg_*.h"), qw(indexing.h toasting.h));
 
 my $oids = Catalog::FindAllOidsFromHeaders(@input_files);

src/include/catalog/unused_oids

 #!/usr/bin/perl
+#----------------------------------------------------------------------
 #
 # unused_oids
+#    Finds blocks of manually-assignable OIDs that have not already been
+#    claimed by previous hackers.  The main use is for finding available
+#    OIDs for new internal functions.  The numbers printed are inclusive
+#    ranges of unused OIDs.
 #
-# src/include/catalog/unused_oids
-#
-#	finds blocks of manually-assignable oids that have not already been
-#	claimed by post_hackers.  primarily useful for finding available
-#	oids for new internal functions.  the numbers printed are inclusive
-#	ranges of unused oids.
-#
-#	before using a large empty block, make sure you aren't about
+#    Before using a large empty block, make sure you aren't about
 #    to take over what was intended as expansion space for something
 #    else.
 #
-#	run this script in src/include/catalog.
+# Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
+# Portions Copyright (c) 1994, Regents of the University of California
 #
-use lib '../../backend/catalog/';
-use Catalog;
+# src/include/catalog/unused_oids
+#
+#----------------------------------------------------------------------
 
 use strict;
 use warnings;
 
+# Must run in src/include/catalog
+use FindBin;
+chdir $FindBin::RealBin or die "could not cd to $FindBin::RealBin: $!\n";
+
+use lib "$FindBin::RealBin/../../backend/catalog/";
+use Catalog;
+
 my @input_files = (glob("pg_*.h"), qw(indexing.h toasting.h));
 
 my $oids = Catalog::FindAllOidsFromHeaders(@input_files);
@@ -45,10 +52,5 @@ foreach my $oid (sort { $a <=> $b } @{$oids})
 			printf "%d\n", $prev_oid + 1;
 		}
 	}
-	elsif ($oid == $prev_oid)
-	{
-		print "Duplicate oid detected: $oid\n";
-		exit 1;
-	}
 	$prev_oid = $oid;
 }