Code review: debian/postinst fix, debian/postrm description + fixes

Sorry about the mutiple git commits in my branch - if someone knows how to clean up, please tell me how, I can't shrink this git rebase loop.

Please note the url mentioned in description of debian/postrm. Is this a good thing, to link documentation URLs pointing to debian.org?

I am not a regex guru but I understood 90% of the regex on postinst. Should I write down an explanation since regex is black magic for many folks?

Thank you for your time!