In contrast to the other service types oneshot services are usually not
long lasting and there's not necessarily a single clean main process for
them. This change allows multiple ExecStart= lines for this type of
services so that the admin/developer doesn't have to arbitrarily pick on
of various sequential commands as the "main one".
This patch gives minimal Arch support (enough to boot).
We still need to parse the services to start from /etc/rc.conf before systemd can be a drop-in replacement on Arch.
