Send server configuration data to client upon login

[congratulatio]

This is for server configuration that would be useful for the client to know. Currently:

Name	Description	Purpose
sf	Scale factor limit	Client may limit or warn when its scale factor exceeds this
afk	AFK disconnection timeout	Client may provide a custom error message

I'm currently testing this on eu.airmash.online game servers and https://test.airmash.online client, but will revert if this PR is not a suitable approach.

[congratulatio]

After reading #85 (comment) I am wondering if this would be more appropriate as a new field in the LOGIN packet. Though, that does limit the configuration update to when the player joins, and in future we may want to expose a server configuration that could be changed during the game, so maybe not.

[wight-airmash]

Yes, I think it's the right place for that kind of data. In addition, SERVER_CUSTOM has the advantage over LOGIN, it can be sent unlimited number of times with updated data, but LOGIN only once.