cgroup-util: do not check validity of controller in cg_split_spec()

cgroup-util: do not check validity of controller in cg_split_spec()

Now the controller part is always ignored, hence let's skip check for
the controller part of the spec. This also make it acceppt unnormalized
path. Previously paths were checked by path_is_normalized(), but now
checked by path_is_safe(). Also, now this mapps an empty path to NULL.