cgroup-util: cache all cg_is_*_wanted answers, disable /sys/fs/cgroups/unified on...

cgroup-util: cache all cg_is_*_wanted answers, disable /sys/fs/cgroups/unified on unified

If we encounter an error in proc cmdline parsing, just treat that as permanent,
i.e. the same as if the option was not specified. Realistically, it is better
to use the same condition for all related mounts, then to have e.g.
/sys/fs/cgroup mounted and /sys/fs/cgroup/unified not. If we find something is
mounted and base our answer on that, cache that result too.

Fix the conditions so that if "unified" is used, make sure any "hybrid" mounts
are not mounted.