Add SQL-accessible functions for inspecting index AM properties.

Per discussion, we should provide such functions to replace the lost
ability to discover AM properties by inspecting pg_am (cf commit
65c5fcd35).  The added functionality is also meant to displace any code
that was looking directly at pg_index.indoption, since we'd rather not
believe that the bit meanings in that field are part of any client API
contract.

As future-proofing, define the SQL API to not assume that properties that
are currently AM-wide or index-wide will remain so unless they logically
must be; instead, expose them only when inquiring about a specific index
or even specific index column.  Also provide the ability for an index
AM to override the behavior.

In passing, document pg_am.amtype, overlooked in commit 473b93287.

Andrew Gierth, with kibitzing by me and others

Discussion: <87mvl5on7n.fsf@news-spur.riddles.org.uk>

src/backend/access/nbtree/nbtutils.c

@@ -2041,3 +2041,29 @@ btoptions(Datum reloptions, bool validate)
 {
 	return default_reloptions(reloptions, validate, RELOPT_KIND_BTREE);
 }
+
+/*
+ *	btproperty() -- Check boolean properties of indexes.
+ *
+ * This is optional, but handling AMPROP_RETURNABLE here saves opening the rel
+ * to call btcanreturn.
+ */
+bool
+btproperty(Oid index_oid, int attno,
+		   IndexAMProperty prop, const char *propname,
+		   bool *res, bool *isnull)
+{
+	switch (prop)
+	{
+		case AMPROP_RETURNABLE:
+			/* answer only for columns, not AM or whole index */
+			if (attno == 0)
+				return false;
+			/* otherwise, btree can always return data */
+			*res = true;
+			return true;
+
+		default:
+			return false;		/* punt to generic code */
+	}
+}