protocol: add commitment fee logic.

Both sides elect a commitment fee, and the lowest is chosen.  That means
you can't game the other side (but if you offer too low, then can error
out of course).

Fees are split 50-50 if possible: originally the whole fee has to be
paid by the (single) funder.  Neither side can withdraw funds which
would make them unable to pay fees.

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

lightning.pb-c.h

@@ -127,6 +127,10 @@ struct  _OpenChannel
    */
   protobuf_c_boolean has_min_depth;
   uint32_t min_depth;
+  /*
+   * How much fee would I like on commitment tx?
+   */
+  uint64_t commitment_fee;
   OpenChannel__LocktimeCase locktime_case;
   union {
     uint32_t locktime_seconds;
@@ -135,7 +139,7 @@ struct  _OpenChannel
 };
 #define OPEN_CHANNEL__INIT \
  { PROTOBUF_C_MESSAGE_INIT (&open_channel__descriptor) \
-    , NULL, NULL, NULL, 0, 0,0u, OPEN_CHANNEL__LOCKTIME__NOT_SET, {} }
+    , NULL, NULL, NULL, 0, 0,0u, 0, OPEN_CHANNEL__LOCKTIME__NOT_SET, {} }
 
 
 /*