funder: don't re-reserve utxos on retries

This way unreserving the PSBT will work as intended, and we don't have
to keep track of how many times we've called reserved for any one input.

Technically we're supposed to not reserve inputs at *all* while doing
opens, this moves us slightly closer to that.

plugins/funder.c

@@ -695,6 +695,8 @@ listfunds_success(struct command *cmd,
 					     committed_funds,
 					     avail_utxos);
 		json_add_bool(req->js, "reservedok", true);
+		/* We don't re-reserve any UTXOS :) */
+		json_add_num(req->js, "reserve", 0);
 	} else {
 		req = jsonrpc_request_start(cmd->plugin, cmd,
 					    "fundpsbt",