Expose capabilities

[ShaneMcC]

Generally, the capabilities we announce to servers as supporting are transparent to the client (eg, it doesn't need to know we support "server-time" or "multi-prefix" etc as it will get timestamped callbacks and ChannelClients will just have all the modes, but some capabilities may change the way the client behaves.

For example, we support "away-notify" as an alternative to polling channel WHO's - it should be possible for the client to see that this capability has been enabled and thus not to schedule it's regular channel WHOs and just wait for notifies.