Use Type=simple and a '-f' to start renderd

Type=forking is not necessary when using systemd. Traditional UNIX
daemons did a fork() call on the main process and terminated the main
process. The PID file was used to identify the forked child.
Using Type=simple will allow systemd to assume the process started via
the ExecStart is the daemons main process, rendering the former
behaviour obsolete.

See http://www.freedesktop.org/software/systemd/man/systemd.service.html
for more information.