lightning/bolts#672

We check the received shutdown script against higher segwit versions and
accept closing to that script if option_shutdown_anysegwit has been
negotiated.

* on channel opening we verify that the peer's dust limit is above 354
  sat, the limit for unknown segwit versions
* we constrain the allowed scriptpubkey types for channel closing
* we check that the remote's output is above the relay dust limit for
  the collaborative close case