Fix fund output calculations

We were doing a couple of things wrong with the `extra_fee`, which is
normally used to account for potential buffer transaction fees when
setting up a DLC channel:

- We were not splitting the cost of the `extra_fee` between both
parties. Instead, we were accounting for it _twice_, deducted in full
from both the change output of the offer party and the accept party.

- We were using the "extra" `extra_fee` to pay more transaction fees
for the funding transaction. This is usually a completely unnecessary
overpayment.

dlc/src/lib.rs

@@ -402,31 +402,43 @@ pub(crate) fn create_fund_transaction_with_fees(
 ) -> Result<(Transaction, Script), Error> {
     let total_collateral = checked_add!(offer_params.collateral, accept_params.collateral)?;
 
+    let (offer_extra_fee, accept_extra_fee) = {
+        let half = extra_fee / 2;
+        let remainder = extra_fee % 2;
+
+        // The offer party has to pay an extra sat if there is a remainder.
+        (half + remainder, half)
+    };
+
     let (offer_change_output, offer_fund_fee, offer_cet_fee) =
-        offer_params.get_change_output_and_fees(fee_rate_per_vb, extra_fee)?;
+        offer_params.get_change_output_and_fees(fee_rate_per_vb, offer_extra_fee)?;
+
     let (accept_change_output, accept_fund_fee, accept_cet_fee) =
-        accept_params.get_change_output_and_fees(fee_rate_per_vb, extra_fee)?;
+        accept_params.get_change_output_and_fees(fee_rate_per_vb, accept_extra_fee)?;
 
     let fund_output_value = checked_add!(offer_params.input_amount, accept_params.input_amount)?
         - offer_change_output.value
         - accept_change_output.value
         - offer_fund_fee
-        - accept_fund_fee
-        - extra_fee;
+        - accept_fund_fee;
 
+    // The fund output has to cover: the total collateral being wagered and the fee reserve to pay
+    // for all transaction fees beyond the funding transaction. In the worst case, this is a buffer
+    // transaction (for DLC channels) and a CET or refund transaction.
     assert_eq!(
-        total_collateral + offer_cet_fee + accept_cet_fee + extra_fee,
-        fund_output_value
+        fund_output_value,
+        total_collateral + offer_cet_fee + accept_cet_fee + offer_extra_fee + accept_extra_fee
     );
 
+    // The sum of the inputs provided by both parties have to cover: the fund output; all the change
+    // outputs; and the funding TX fee.
     assert_eq!(
         offer_params.input_amount + accept_params.input_amount,
         fund_output_value
             + offer_change_output.value
             + accept_change_output.value
             + offer_fund_fee
             + accept_fund_fee
-            + extra_fee
     );
 
     let fund_sequence = util::get_sequence(fund_lock_time);