cleanup(sys-apps/systemd): Remove unused files.

These are installed by gentoo-systemd-integration these days.
fix(sys-apps/systemd): Don't prune systemd dirs

Assorted config directories in /etc are normally empty but without them
users generally have no idea that they can actually use them. Switch to
installing them again and add tmpfiles rules to recreate them.
Merge pull request #550 from marineam/systemd

Minor systemd fixes