Have the client pass its stdin fd to the server when identifying itself and

have the server use that rather than reopening the tty. If the fd isn't given,
use the old behaviour (so no need for a version change).

This allows tmux to be used as the shell, so also change so that when working
out the command to execute if default-command is empty (the default), tmux will
try not execute itself.

client.c

@@ -119,7 +119,8 @@ server_started:
 		if (strlcpy(data.tty, name, sizeof data.tty) >= sizeof data.tty)
 			fatalx("ttyname failed");
 
-		client_write_server(cctx, MSG_IDENTIFY, &data, sizeof data);
+		imsg_compose(&cctx->ibuf, MSG_IDENTIFY,
+		    PROTOCOL_VERSION, -1, STDIN_FILENO, &data, sizeof data);
 	}
 
 	return (0);