Remove everything GPL that is not copyright Guus Sliepen, update copyright statements.

Thanks to faithful use of revision control systems, especially git, it has been
relatively easy to find those parts of tinc that were contributed by others. A
large part of that code was not useful for the MeshLink library anyway
(advanced configuration options, privilege dropping and chrooting), the
remaining features contributed by others have been removed (most notably
Brandon Black's improvements related to UDP packet loss and reordering, Etienne
Dechamps' type 2 MTU probe replies, Florian Forster's interface binding code,
Michal Tokarev's support for tunnelserver mode). Support for LZO has been
removed, since it is a GPL library (unless a separate license agreement with
Oberhumer.com is made).