Support the `channel_type` feature bit.

Support the `channel_type` feature bit.

Note that this feature bit does absolutely nothing. We signal it
(as we already support channel type negotiation), but do not bother
to look to see if peers support it, as we don't care - we simply
look for the TLV entry and deduce if a peer supports channel type
negotiation from that.

The only behavioral change at all here is that we don't barf if a
peer sets channel type negotiation to required via the feature bit
(instead of failing the channel at open-time), but of course no
implementations do this, and likely won't for some time (if ever -
you can simply fail channels with unknown types later, and there's
no reason to refuse connections, really).

As defined in https://github.com/lightning/bolts/pull/906