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/include/nodes/parsenodes.h

@@ -1776,6 +1776,7 @@ typedef struct CreateOpClassItem
 	List	   *name;			/* operator or function name */
 	List	   *args;			/* argument types */
 	int			number;			/* strategy num or support proc num */
+	List	   *order_family;	/* only used for ordering operators */
 	List	   *class_args;		/* only used for functions */
 	/* fields used for a storagetype item: */
 	TypeName   *storedtype;		/* datatype stored in index */