Rework debian/, following current best practices

* Switch to minimal debhelper approach, Build-Depend on debhelper >= 10
* Switch from Priority 'extra' to 'optional' (deprecated as of Debian Policy v4.0.1)
* Drop unused GPL-2 section from debian/copyright
* Update copyright year information in debian/copyright
* Refresh lintian overrides
* Add postinst/postrm maintainer scripts for dpkg-maintscript-helper support