light-locker: use systemd/logind instead of UPower

Since systemd has been adopted for a while now, we should switch to
using it for light-locker as well. So I disabled ConsoleKit/UPower
support in favor of using systemd with logind. This fixed many issues
for me, and made light-locker working again.

I followed the PKGBUILD of Arch's package in determining the right
configure flags. See: https://git.archlinux.org/svntogit/community.git/tree/trunk/PKGBUILD?h=packages/light-locker