Fixes: #98
Add a Kill method for cgroups v2. This uses the new functionality in
kernels 5.14+ where writing to a cgroup.kill file will send a SIGKILL
to every process running in the cgroup. This would be useful for
kata to avoid freezing+manually sending SIGKILL+thawing process it
does currently to emulate runc's behavior.
Signed-off-by: Danny Canter <danny@dcantah.dev>