Support both myroomnick and roomnick

It's a more common-used command and it seems all other clients uses /myroomnick as the command name, so we will no longer need to state two answers like "You can use /roomnick if you use NeoChat or use /myroomnick if you use other client to change your display name in this room" when answering the question about how to change user's room nick.