sort: show warning when sort is not supported

Use the capabilities returned by the backend to check whether sort is
implemented when the user tries to use the sort command. Print a warning
to the log when a sort request is silently dropped by the backend.

Suggested-by: |cos|
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/msgstore.go

@@ -847,3 +847,8 @@ func (store *MessageStore) FindIndexByUid(uid uint32) int {
 	}
 	return -1
 }
+
+// Capabilities returns a models.Capabilities struct or nil if not available
+func (store *MessageStore) Capabilities() *models.Capabilities {
+	return store.DirInfo.Caps
+}