core: Make (user) instance aware of delegated cgroup controllers

core: Make (user) instance aware of delegated cgroup controllers

systemd user instance assumed same controllers are available to it as to
PID 1. That is not true generally, in v1 (legacy, hybrid) we don't delegate any
controllers to anyone and in v2 (unified) we may delegate only subset of
controllers.
The user instance would fail silently when the controller cgroup cannot
be created or the controller cannot be enabled on the unified hierarchy.

The changes in 7b63961415 ("cgroup: Swap cgroup v1 deletion and
migration") caused some attempts of operating on non-delegated
controllers to be logged.

Make the user instance first check what controllers are availble to it
and narrow operations only to these controllers. The original checks are
kept in place.

Note that daemon-reexec needs to be invoked in order to update the set
of unabled controllers after a change.

Fixes: #18047
Fixes: #17862