Replace simple constant pg_am.amcanreturn with an AM support function.

The need for this was debated when we put in the index-only-scan feature,
but at the time we had no near-term expectation of having AMs that could
support such scans for only some indexes; so we kept it simple.  However,
the SP-GiST AM forces the issue, so let's fix it.

This patch only installs the new API; no behavior actually changes.

src/backend/access/index/indexam.c

@@ -26,6 +26,7 @@
  *		index_getbitmap - get all tuples from a scan
  *		index_bulk_delete	- bulk deletion of index tuples
  *		index_vacuum_cleanup	- post-deletion cleanup of an index
+ *		index_can_return	- does index support index-only scans?
  *		index_getprocid - get a support procedure OID
  *		index_getprocinfo - get a support procedure's lookup info
  *
@@ -711,6 +712,27 @@ index_vacuum_cleanup(IndexVacuumInfo *info,
 	return result;
 }
 
+/* ----------------
+ *		index_can_return - does index support index-only scans?
+ * ----------------
+ */
+bool
+index_can_return(Relation indexRelation)
+{
+	FmgrInfo   *procedure;
+
+	RELATION_CHECKS;
+
+	/* amcanreturn is optional; assume FALSE if not provided by AM */
+	if (!RegProcedureIsValid(indexRelation->rd_am->amcanreturn))
+		return false;
+
+	GET_REL_PROCEDURE(amcanreturn);
+
+	return DatumGetBool(FunctionCall1(procedure,
+									  PointerGetDatum(indexRelation)));
+}
+
 /* ----------------
  *		index_getprocid
  *