Allow to bind to privileged port (sudo)

Based on https://github.com/BerlinVagrant/vagrant-dns/pull/31

This generall works fine.

The only edge case which is problematic is when changing the port from a privileged to a non-privileged one.
Then I'll try stopping the previous daemon process (which is running as `root`) as the current user, which obviously fails.
To work around that we could always use `sudo`, but this might annoy people who are not using privileged ports.

@kris-lab @fnordfish please review