Preconnect server hook

Calls ClientPreConnect on SVQC when a connection with a client has been established.

This is triggered as soon as a client connects to the engine and its entity is initialized.

Useful to add some customized logging which in turn can be used by external scripts to take some actions, like more sophisticated banning.

On the QuakeC side, when ClientPreConnect is called, self will be set to the client entity and it will only have basic information set, like netaddress and crypto keys.