cgroup: Add EffectiveMemoryMax=, EffectiveMemoryHigh= and EffectiveTasksMax= properties

cgroup: Add EffectiveMemoryMax=, EffectiveMemoryHigh= and EffectiveTasksMax= properties

Users become perplexed when they run their workload in a unit with no
explicit limits configured (moreover, listing the limit property would
even show it's infinity) but they experience unexpected resource
limitation.

The memory and pid limits come as the most visible, therefore add new
unit read-only properties:
- EffectiveMemoryMax=,
- EffectiveMemoryHigh=,
- EffectiveTasksMax=.

These properties represent the most stringent limit systemd is aware of
for the given unit -- and that is typically(*) the effective value.

Implement the properties by simply traversing all parents in the
leaf-slice tree and picking the minimum value. Note that effective
limits are thus defined even for units that don't enable explicit
accounting (because of the hierarchy).

(*) The evasive case is when systemd runs in a cgroupns and cannot
reason about outer setup. Complete solution would need kernel support.