[kwsantiago]

Summary

A local channel funded before the BLAKE2b activation can never be announced, but derive_channel_state put it in CGOSSIP_ANNOUNCED when its announce flag was set. On every start lightningd logged "gossipd lost track of announced channel", rebroadcast its channel_update to every peer, and kept signing a node_announcement that upgraded nodes drop.

• is_private now also covers a usable channel whose scid predates the activation, so it is handled like an unannounced channel: its channel_update goes only to its peer, it does not count toward has_announced_channels, and stale peer sigs are dropped on entering CGOSSIP_PRIVATE.
• New transitions for the paths into and out of that state: going live, a zeroconf funding mined below the activation (also while closing), a zeroconf funding reorganized out, and a splice above the activation, which takes the channel back to the public path.
• announcement_signatures for such a channel are ignored quietly, since the peer may not know the activation height.
• On start, gossipd still hands back the channel_update of a closed channel funded before the activation. It is now dropped instead of tripping the state checks.

Closes #80

Test plan

• test_gossip_prefork_channel_not_reannounced: two restarts start in CGOSSIP_PRIVATE with no re-announce, a new alias is never announced, and invoices still carry a route hint that a third node pays through.
• test_gossip_prefork_channel_closed_restart and test_gossip_prefork_zeroconf_closing_mined fail without the change (a BROKEN log and an illegal transition) and pass with it.
• The existing activation tests, including the splice above the activation, pass.
• Locally: test_gossip, test_invoices, test_splicing, test_blake2b_differentiation and test_closing on v1, plus test_gossip, test_splicing and test_closing with dual funding.

Changelog-None