protocol: no signature in update_commit if receiver has no outputs.

So if there are no HTLCs, and the receiver can't spend anyway, don't
sign.  This has the added benefit that no two signed commitment
transactions will ever be identical (the revocation preimage changes).

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

daemon/commit_tx.h

@@ -34,5 +34,6 @@ struct bitcoin_tx *create_commit_tx(const tal_t *ctx,
 				    struct peer *peer,
 				    const struct sha256 *rhash,
 				    const struct channel_state *cstate,
-				    enum htlc_side side);
+				    enum htlc_side side,
+				    bool *otherside_only);
 #endif