add needed empty defines

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
cgroup: move all some functions into cgroup.h

Some functions which wanted to know about cgroup paths were located
in other files.  Move them into cgroup.c, so that all knowledge of
the cgroup backend can be colocated.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>