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/spgist/spgscan.c

@@ -559,3 +559,10 @@ spggettuple(PG_FUNCTION_ARGS)
 
 	PG_RETURN_BOOL(false);
 }
+
+Datum
+spgcanreturn(PG_FUNCTION_ARGS)
+{
+	/* Not implemented yet */
+	PG_RETURN_BOOL(false);
+}