socket: add support for selecting interface again

Revert commit e49aececce ("socket: don't set interface for sent
packets") to allow the interface to be selected for outgoing packets,
but don't set it in the callers yet.

cmdmon.c

@@ -301,6 +301,9 @@ transmit_reply(int sock_fd, SCK_Message *message)
 {
   message->length = PKL_ReplyLength((CMD_Reply *)message->data);
 
+  /* Don't require the response to use the same interface */
+  message->if_index = INVALID_IF_INDEX;
+
   if (!SCK_SendMessage(sock_fd, message, 0))
     return;
 }