libprocessgroup: Rework SetupCgroup()

Improve the readability of this function by splitting it.

This CL includes the following behavior changes:
- If changing the directory owner and/or mode fails for /sys/fs/cgroup,
  this is considered as a fatal error instead of something that should
  only fail if "Optional" has not been set.
- If mounting the v2 cgroup controller fails, this is considered as an
  error.
- Activating/mounting a cgroup controller only fails if the controller
  has not been marked as optional.

Bug: 213617178
Change-Id: If6908dfdbcb2e1c9637ab4ac8a7625f0a17dc9e0
Signed-off-by: Bart Van Assche <bvanassche@google.com>