Expose a public registration hook so external packages that register a channel
factory via channels.RegisterFactory can also register the channel's settings
struct prototype. Without this, InitChannelList rejects any channel type absent
from the private channelSettingsFactory map, making out-of-tree channels
impossible without forking. The map access is now guarded by a RWMutex.
This is additive and changes no existing behavior.
Carlos Prados
e55ddf2e24