cgfsng: add cgfsng_monitor_destroy()

cgfsng: add cgfsng_monitor_destroy()

Since we switched to the new cgroup scoping scheme that places the
container payload into lxc.payload/<container-name> and
lxc.monitor/<container-name> deletion becomes slightly more complicated.
The monitor will be able to rm_rf(lxc.payload/<container-name>) but will
not be able to rm_rf(lxc.monitor/<container-name>) since it will be
located in that cgroup and it will thus be populated.
My current solution to this is to create a lxc.pivot cgroup that only
exists so that the monitor process on container stop can pivot into it,
call rm_rf(lxc.monitor/<container-name>) and can then exit. This group
has not function whatsoever apart from this and can thus be shared by
all monitor processes.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>