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/nodes/equalfuncs.c

@@ -1464,6 +1464,7 @@ _equalCreateOpClassItem(CreateOpClassItem *a, CreateOpClassItem *b)
 	COMPARE_NODE_FIELD(name);
 	COMPARE_NODE_FIELD(args);
 	COMPARE_SCALAR_FIELD(number);
+	COMPARE_NODE_FIELD(order_family);
 	COMPARE_NODE_FIELD(class_args);
 	COMPARE_NODE_FIELD(storedtype);