lightningd: mark subd->conn notleak() properly in transition.

Since subd is temporarily unreferenced, and our leak detection goes up
to the highest unreferenced parent, it complains about openingd->conn.

The correct fix is to catch this where we detach the subd:

```
DEBUG:root:{
  "id": 1,
  "result": {
    "leaks": [
      {
        "value": "0x556e0445d8f8",
        "label": "ccan/ccan/io/io.c:91:struct io_conn",
        "backtrace": [
          "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
          "ccan/ccan/io/io.c:91 (io_new_conn_)",
          "lightningd/subd.c:774 (new_subd)",
          "lightningd/subd.c:828 (new_channel_subd_)",
          "lightningd/opening_control.c:872 (peer_start_openingd)",
          "lightningd/peer_control.c:1311 (peer_active)",
          "lightningd/connect_control.c:458 (connectd_msg)",
          "lightningd/subd.c:557 (sd_msg_read)",
          "lightningd/subd.c:357 (read_fds)",
          "ccan/ccan/io/io.c:59 (next_plan)",
          "ccan/ccan/io/io.c:407 (do_plan)",
          "ccan/ccan/io/io.c:417 (io_ready)",
          "ccan/ccan/io/poll.c:453 (io_loop)",
          "lightningd/io_loop_with_timers.c:22 (io_loop_with_timers)",
          "lightningd/lightningd.c:1182 (main)",
          "../csu/libc-start.c:308 (__libc_start_main)"
        ],
        "parents": [
          "lightningd/lightningd.c:107:struct lightningd"
        ]
      }
    ]
  }
}
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/opening_control.c

@@ -563,6 +563,11 @@ static void openingd_failed(struct subd *openingd, const u8 *msg,
 {
 	char *desc;
 
+	/* Since we're detaching from uc, we'll be unreferenced until
+	 * our imminent exit (as will our parent, openingd->conn). */
+	notleak(openingd);
+	notleak(openingd->conn);
+
 	if (!fromwire_openingd_failed(msg, msg, &desc)) {
 		log_broken(uc->log,
 			   "bad OPENINGD_FAILED %s",
@@ -676,8 +681,6 @@ openchannel_hook_final(struct openchannel_hook_payload *payload STEALS)
 	} else
 		upfront_shutdown_script_wallet_index = NULL;
 
-	/* In case peer goes away right now, mark openingd notleak() */
-	notleak(openingd);
 	subd_send_msg(openingd,
 		      take(towire_openingd_got_offer_reply(NULL, errmsg,
 							   our_upfront_shutdown_script,