Rework debian/ to follow current best practices

* Switch to minimal debhelper approach
* Provide debian/source/format
* Bump Standards-Version to 4.5.1
* Switch all Homepage and Vcs-* headers to
  https://github.com/grml/grml-network (this avoids having a strange
  mixture of our self-hosted git repositories and github and
  also fixes vcs-field-uses-insecure-uri lintian warning)
* Convert debian/copyright to machine-readable format

Gbp-Dch: Full