systemd: always use /lib/systemd

Debian and Ubuntu use /lib/systemd whereas Fedora usr /usr/lib/systemd as the
default system unit directory. This already leads to conflict when the
debian-systemd is used.

Fedora provides a symbolic link between /lib/systemd and /usr/lib/systemd
to also support /lib/systemd directory. With this patch, we use /lib/systemd
everywhere.

This patch supersedes I413a92284e6a79e7fcdde99c5138bc3ce8e85e80.

Change-Id: I459f7514ab35082d31607968252a9005fa25de2a
See: https://bugs.debian.org/719634
Merge "systemd: always use /lib/systemd"