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>

test-cli/create-commit-tx.c

@@ -16,6 +16,7 @@
 #include "find_p2sh_out.h"
 #include "protobuf_convert.h"
 #include "gather_updates.h"
+#include "funding.h"
 #include <unistd.h>
 
 /* FIXME: this code doesn't work if we're not the ones proposing the delta */
@@ -65,7 +66,8 @@ int main(int argc, char *argv[])
 
 	sig2.stype = SIGHASH_ALL;
 
-	gather_updates(o1, o2, a, argv + 5, &our_amount, &their_amount,
+	gather_updates(o1, o2, a, commit_fee(o1, o2), argv + 5,
+		       &our_amount, &their_amount,
 		       &rhash, NULL, &sig2.sig);
 
 	redeemscript = bitcoin_redeem_2of2(ctx, &pubkey1, &pubkey2);