Allow to bind to privileged port (sudo)

[njam]

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

[njam]

I've found a better solution for this problem: Installing a local bind DNS server which forwards to the vagrant-dns server with:

options {
    forwarders {
        127.0.0.1 port 5300;
    };
};