Draft: Install systemd files into /usr/lib/systemd/ (closes: #993958)

With debhelper now installing systemd service files and timers into /usr/lib/systemd/system/ instead of /lib/systemd/system/, apt should also install those files under /usr/. This prevents the appearence of an ampty /lib/systemd/ in .deb package.

This commit fixes Debian bug #993958 .