The cgroupfs-specific code is moved from attach.c to cgroup.c.

lxc-cgmanager now only chgrps the container's cgroup, so that the
unprivileged user still owns the tasks file allowing him to enter
the container cgroup (for attach).

Some other changes rolled into the cgmanager update:

Make the list of subsystems not per-handler, as it will not change.  As
a result, the only state we need to keep in the per-handler cgroup data
is the char *cgroup_path, so we can drop the cgm_data struct altogether.

Catch nih errors (as not doing so causes later crashes).
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>