fix(tmpfiles): auto-generate tmpfiles based on .keep

To avoid having to sync directory creation and tmpfiles installed by
ebuilds just require ebuilds to do `keepdir /etc/path` and this script
will handle the rest. A possible extension would be to update to handle
symlinks in addition to directories in gen_tmpfiles.py
Merge pull request #157 from marineam/etc-dirs

fix(tmpfiles): auto-generate tmpfiles based on .keep