Add NotifyStartedFunc field to Server

Adds a field, NotifyStartedFunc func() to Server.

If non-nil, it is called after a server starts listening. This is useful
for synchronization purposes, for example when a daemon needs to drop
privileges after binding. Otherwise, there is no way to determine when
the server has begun listening and hardcoded delays (!) must be used or
race conditions may occur.
Merge pull request #159 from hlandauf/racefix

Add NotifyStartedFunc field to Server