lightningd: add withhold option to fundchannel_complete.

This is just a polite way of telling us that if we close, don't bother broadcasting
since we didn't broadcast the funding tx.

Changelog-Added: JSON-RPC: `fundchannel_complete` new parameter `withhold` (default false).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/schemas/fundchannel_complete.json

@@ -26,6 +26,13 @@
         "description": [
           "Transaction to use for funding (does not need to be signed but must be otherwise complete)."
         ]
+      },
+      "withhold": {
+        "type": "boolean",
+        "added": "v25.12",
+        "description": [
+          "Mark this channel 'withheld' so we know we haven't broadcast the funding transaction.  If the channel is closed before we call `sendpsbt` on this psbt, it will simply be closed immediately."
+        ]
       }
     }
   },