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/include/access/nbtree.h

@@ -748,6 +748,9 @@ extern void _bt_end_vacuum_callback(int code, Datum arg);
 extern Size BTreeShmemSize(void);
 extern void BTreeShmemInit(void);
 extern bytea *btoptions(Datum reloptions, bool validate);
+extern bool btproperty(Oid index_oid, int attno,
+		   IndexAMProperty prop, const char *propname,
+		   bool *res, bool *isnull);
 
 /*
  * prototypes for functions in nbtvalidate.c