changes to debian packaging for balsa NMU

I've just pushed an NMU of balsa 2.5.9 to experimental.

This branch reflects those changes (as do the upstream/latest and pristine-tar branches you'll find in this fork).

I'm currently trying to get balsa in better shape upstream (as part of the gmime transition and my general interest in there being functional/decent MUAs available). If you want to grant me push access to the salsa balsa repo, i'm happy to help out more directly.