core: when Delegate=yes is set for a unit, run ExecStartPre= and friends in a subcgro...

core: when Delegate=yes is set for a unit, run ExecStartPre= and friends in a subcgroup of the unit

Otherwise we might conflict with the "no-processes-in-inner-cgroup" rule
of cgroupsv2. Consider nspawn starting up and initializing its cgroup
hierarchy with "supervisor/" and "payload/" as subcgroup, with itself
moved into the former and the payload into the latter. Now, if an
ExecStartPre= is run right after it cannot be placed in the main cgroup,
because that is now in inner cgroup with populated children.

Hence, let's run these helpers in another sub-cgroup .control/ below it.

This is somewhat ugly since it weakens the clear separation of
ownership, but given that this is an explicit contract, and double opt-in should be acceptable.

Fixes: #10482