core: always use "infinity" for no upper limit instead of "max" (#3417)

core: always use "infinity" for no upper limit instead of "max" (#3417)

Recently added cgroup unified hierarchy support uses "max" in configurations
for no upper limit.  While consistent with what the kernel uses for no upper
limit, it is inconsistent with what systemd uses for other controllers such as
memory or pids.  There's no point in introducing another term.  Update cgroup
unified hierarchy support so that "infinity" is the only term that systemd
uses for no upper limit.