common/sphinx: don't use fixed lengths anywhere.

1. Remove the very concept of ONION_REPLY_SIZE, instead make it a
   local variable in create_onionreply().

2. Use the proper fromwire_ primitives in unwrap_onionreply() so we
   don't have to do explicit length checks.

3. Make fromwire_tal_arrn() return NULL if it fails to pull, instead of
   a zero-length allocation.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: Protocol: we now correctly decrypt non-256-length onion errors (we always forwarded them fine, now we actually can parse them).

tests/test_plugin.py

@@ -1042,7 +1042,6 @@ def test_channel_state_change_history(node_factory, bitcoind):
         assert(history[3]['message'] == "Closing complete")
 
 
-@pytest.mark.xfail(strict=True)
 @pytest.mark.developer("Gossip slow, and we test --dev-onion-reply-length")
 def test_htlc_accepted_hook_fail(node_factory):
     """Send payments from l1 to l2, but l2 just declines everything.