introduce lxc.cgroup.dir.{monitor,container,container.inner}

introduce lxc.cgroup.dir.{monitor,container,container.inner}

This is a new approach to #1302 with a container-side
configuration instead of a global boolean flag.

Contrary to the previous PR using an optional additional
parameter for the get-cgroup command, this introduces two
new additional commands to get the limiting cgroup path and
cgroup2 file descriptor. If the limiting option is not in
use, these behave identical to their full-path counterparts.

If these variables are used the payload will end up in the
concatenation of lxc.cgroup.dir.container and
lxc.cgroup.dir.container.inner (which may be empty), and the
monitor will end up in lxc.cgruop.dir.monitor. The
directories are fixed, no retry count logic is applied,
failing to create these directories will simply be a hard
error.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>