engine: route popup preedit by client capability

srv.c

@@ -41,6 +41,7 @@ clientthread(void *arg)
 	reply = chancreate(sizeof(Keyres), 0);
 	kr.reply = reply;
 	kr.owner = &fd;
+	kr.cap = Cclientpreedit;
 	while((want = srvreadkey(fd, &kr)) >= 0){
 		chansend(keyc, &kr);
 		chanrecv(reply, &res);