cgroup: allow to specify a new root for each controller separately

A few controllers can be merged and before this patch, criu required to
set a new root by specifying all merged controllers.

This patch allows to specify a new root for each controller separately,
but criu will check that all merged controllers will have the same path.

https://github.com/checkpoint-restore/criu/issues/544



Signed-off-by: Andrei Vagin <avagin@gmail.com>