move cgroup.c to cgfs.c in preparation for backend change

This is just a move without any changes so history will be preserved.
Makefile.am was modified so that lxc will still build and run.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>