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
Small cleanup of the docs
Tests: fix TestShutdownTCP race using NotifyStartedFunc
Merge pull request #160 from FiloSottile/fix_tcp_test_race

Tests: fix TestShutdownTCP race using NotifyStartedFunc
dns.Msg.Unpack with mismatch in bytes is now an error

Unpack would see that the offset did not match the length of the input buffer.
We had commented out the println to report this. It is now an error.
Fix import paths