{GPL, other}: Relicense to MIT license.

This change relicenses all files where the sole owner(s) are Sebastian
Harl and/or Florian Forster to the MIT license. This change happens
according to prior agreement with Sebastian.
collectd-tg: Fix license in usage output.
Also relicense all of Pierre-Yves Ritschard's work to the MIT license.
bindings/perl/lib/Collectd/Unixsock.pm: Relicense to MIT.