Create the system catalog infrastructure needed for KNNGIST.

This commit adds columns amoppurpose and amopsortfamily to pg_amop, and
column amcanorderbyop to pg_am.  For the moment all the entries in
amcanorderbyop are "false", since the underlying support isn't there yet.

Also, extend the CREATE OPERATOR CLASS/ALTER OPERATOR FAMILY commands with
[ FOR SEARCH | FOR ORDER BY sort_operator_family ] clauses to allow the new
columns of pg_amop to be populated, and create pg_dump support for dumping
that information.

I also added some documentation, although it's perhaps a bit premature
given that the feature doesn't do anything useful yet.

Teodor Sigaev, Robert Haas, Tom Lane

src/backend/utils/cache/syscache.c

@@ -135,11 +135,11 @@ static const struct cachedesc cacheinfo[] = {
 	},
 	{AccessMethodOperatorRelationId,	/* AMOPOPID */
 		AccessMethodOperatorIndexId,
-		2,
+		3,
 		{
 			Anum_pg_amop_amopopr,
+			Anum_pg_amop_amoppurpose,
 			Anum_pg_amop_amopfamily,
-			0,
 			0
 		},
 		64