This replaces OOMAdjust= by OOMScoreAdjust= in the config files,
breaking compatibility with older unit files. However, this keeps compat
with older kernels which lack the new OOM rework.
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".
@ makes more sense since the NUL addr contained in the sockaddr's
sun_path field can also be written as ^@.
Also, in the .socket files writing "ListenStream==foo" is kinda ugly.
Finally, tools like strace decode it with an @ too.
