cgroups: Export ParseCgroupFile

**Note**: I'm rebasing Josh Poimboeuf's libvirt3 branch on top of docker master, and this commit of his branch would be needed in libcontainer to avoid code duplicates.

This is needed for the libvirt exec driver.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)