network: change license of examples to MIT-0

network: change license of examples to MIT-0

Arguably, CC0 is just fine for examples since they are not code. But it's
easier to be consistent and just use MIT-0 for all "documentation". Thus,
the license is changed similarly code examples under man/.

Based on 'git shortlog -ns network/*' and 'git log -p', the following folks
should ack this:
  Zbigniew Jędrzejewski-Szmek
  Lennart Poettering
  Tom Gundersen
  Yu Watanabe
  Daan De Meyer
  Marc-André Lureau