Support index INCLUDE in the AM properties interface.

This rectifies an oversight in commit 8224de4f4, by adding a new
property 'can_include' for pg_indexam_has_property, and adjusting the
results of pg_index_column_has_property to give more appropriate
results for INCLUDEd columns.

src/include/access/amapi.h

@@ -50,7 +50,8 @@ typedef enum IndexAMProperty
 	AMPROP_CAN_ORDER,			/* AM properties */
 	AMPROP_CAN_UNIQUE,
 	AMPROP_CAN_MULTI_COL,
-	AMPROP_CAN_EXCLUDE
+	AMPROP_CAN_EXCLUDE,
+	AMPROP_CAN_INCLUDE
 } IndexAMProperty;
 
 
@@ -196,6 +197,12 @@ typedef struct IndexAmRoutine
 	/* type of data stored in index, or InvalidOid if variable */
 	Oid			amkeytype;
 
+	/*
+	 * If you add new properties to either the above or the below lists, then
+	 * they should also (usually) be exposed via the property API (see
+	 * IndexAMProperty at the top of the file, and utils/adt/amutils.c).
+	 */
+
 	/* interface functions */
 	ambuild_function ambuild;
 	ambuildempty_function ambuildempty;