Possible alternative to #14819.
For me, setting RemainAfterExit=yes would be OK, but if people think that it
might cause issues, then this could be a reasonable alternative that still
let's us skip the invocation of the separate binary.
Let's use uppercase wording in the description string, like we usually
do.
Let's allow using this service in early boot.
If it's pulled into the initial transaction it's better to finish
loading this before sysinit.target.
Don't bother with this in containers that lack CAP_SYS_MODULE
Devices referred to by `DeviceAllow=` sandboxing are resolved into their
corresponding major numbers when the unit is loaded by looking at
`/proc/devices`. If a reference is made to a device which is not yet
available, the `DeviceAllow` is ignored and the unit's processes cannot
access that device.
In both logind and nspawn, we have `DeviceAllow=` lines, and `modprobe`
in `ExecStartPre=` to load some kernel modules. Those kernel modules
cause device nodes to become available when they are loaded: the device
nodes may not exist when the unit itself is loaded. This means that the
unit's processes will not be able to access the device since the
`DeviceAllow=` will have been resolved earlier and denied it.
One way to fix this would be to re-evaluate the available devices and
re-apply the policy to the cgroup, but this cannot work atomically on
cgroupsv1. So we fall back to a second approach: instead of running
`modprobe` via `ExecStartPre`, we move this out to a separate unit and
order it before the units which want the module.
Closes#14322.
Fixes: #13943.
Zbigniew Jędrzejewski-Szmek