Debian packaging cleanup

[Daniel Kahn Gillmor]

This series offers a set of simple and small changes to the debian
packaging for notmuch.  they apply to the master branch.

I've reviewed these changes, and tested a build with them with no
problems.

A modern, canonicalized debian package is easier to work with.

Please consider applying these changes!  If you're uncomfortable with
any of them, I'm happy to hear feedback.

        --dkg