Rework debian/ to follow current best practices and drop deprecated symlinks

Rework debian/ to follow current best practices and drop deprecated symlinks

* Switch to minimal debhelper approach (which also drops the
  the deprecated /etc/rc*.d/important_notice and /etc/init.d/grml-halt symlinks)
  and use Makefile
* Switch all Homepage and Vcs-* headers to
  https://github.com/grml/grml-etc-core/ (this avoids having a strange
  mixture of our self-hosted git repositories and github and
  also fixes vcs-field-uses-insecure-uri lintian warning)
* Drop debian/overrides and provide up2date debian/grml-etc.lintian-overrides
* Bump Standards-Version to 4.5.1
* Convert debian/copyright to machine-readable format