This removes the ability to configure which cgroup controllers to mount
together. Instead, we'll now hardcode that "cpu" and "cpuacct" are
mounted together as well as "net_cls" and "net_prio".

The concept of mounting controllers together has no future as it does
not exist to cgroupsv2. Moreover, the current logic is systematically
broken, as revealed by the discussions in systemd#10507. Also, we surveyed Red
Hat customers and couldn't find a single user of the concept (which
isn't particularly surprising, as it is broken...)

This reduced the (already way too complex) cgroup handling for us, since
we now know whenever we make a change to a cgroup for one controller to
which other controllers it applies.

… mask according to cpu/cpuacct joint mounting

Note that for cgroup_context_get_mask() this doesn't actually change
much, but it does prepare the ground for systemd#10507 later on.

… take jointly mounted controlelrs into account

If we create a cgroup in one controller it might already have been
created in another too, if we have jointly mounted controllers. Take
that into consideration.