Split ServerEventListener into {Broadcast,Link,Sync}EventListener.•••There is no reason to have these events in one big handler and it
causes unnecessary event hooks to be created by having them like
this.
The ServerEventListener class still exists for compatibility
Improve UserManager::QuitUser() and related code•••- Make operreason optional; NULL means same as quitreason
- Remove User::quietquit, it is now handled internally in spanningtree
- Send snotice about quitting remote users from spanningtree