Fix: systemd: improve unit files

Per http://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/,
"network.target is a passive unit ... Services using the network should hence
simply place an After=network.target dependency in their unit files, and avoid
any Wants=network.target or even Requires=network.target."

I also considered using network-online.target instead of network.target,
but: "It is strongly recommended not to pull in this target too liberally: for
example network server software should generally not pull this in (since server
software generally is happy to accept local connections even before any
routable network interface is up), its primary purpose is network client
software that cannot operate without network."