Show specific OID suggestion in unused_oids output.

Commit a6417078 established a new project policy around OID assignment:
new patches are encouraged to choose a random OID in the 8000..9999
range when a manually-assigned OID is required (if multiple OIDs are
required, a consecutive block of OIDs starting from the random point
should be used).  Catalog entries added by committed patches that use
OIDs from this "unstable" range are renumbered after feature freeze.
This practice minimizes OID collisions among concurrently-developed
patches.

Show a specific random OID suggestion when the unused_oids script is
run.  This makes it easy for patch authors to use a random OID from the
unstable range, per the new policy.

Author: Julien Rouhaud, Peter Geoghegan
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/CAH2-WzkkRs2ScmuBQ7xWi7xzp7fC1B3w0Nt8X+n4rBw5k+Z=zA@mail.gmail.com

src/include/catalog/unused_oids

@@ -38,7 +38,8 @@ my $FirstGenbkiObjectId =
 push @{$oids}, $FirstGenbkiObjectId;
 
 my $prev_oid = 0;
-foreach my $oid (sort { $a <=> $b } @{$oids})
+my @sortedoids = sort { $a <=> $b } @{$oids};
+foreach my $oid (@sortedoids)
 {
 	if ($oid > $prev_oid + 1)
 	{
@@ -53,3 +54,25 @@ foreach my $oid (sort { $a <=> $b } @{$oids})
 	}
 	$prev_oid = $oid;
 }
+
+my $suggestion;
+do
+{
+	$suggestion = int(8000 + rand(2000));
+} while (grep(/^$suggestion$/, @{$oids}));
+
+my $navailable = 0;
+foreach my $oid (@sortedoids)
+{
+	if ($oid > $suggestion)
+	{
+		$navailable = $oid - $suggestion;
+		last;
+	}
+}
+
+printf "Patches should use a more-or-less consecutive range of OIDs.\n";
+printf
+  "Best practice is to start with a random choice in the range 8000-9999.\n";
+printf
+  "Suggested random unused OID: $suggestion ($navailable consecutive OID(s) available starting here)\n";