improve systemd support; install to /sbin

This patchset improves the systemd service, including a fix for issue #13.

It also changes the default installation directory for the daemon to ${sbindir}. If your distribution does not distinguish between /usr/bin and /usr/sbin, overriding that is easy - `./configure --sbindir='${bindir}'` - but distinguishing between the two in the upstream build system is useful for distributions that *do* distinguish, like Debian and Ubuntu.