cgroup: Fix MemoryAvailable= by considering physical memory

cgroup: Fix MemoryAvailable= by considering physical memory

Currently, querying a unit's available memory would result in infinity
if there are no limits set on the unit or ancestors.
That undermines semantics implied by the name, so look at the physical
memory if the search propagates up to the -.slice.
This makes sense even in systemd user instances, limits of -.slice are
still looked at too.

Also change printed representation of infinite MemoryAvailable which
means we could not figure out a good estimate.