listkeys: take a cgroup as argument

freezer:/ has a different list of files than freezer:/foo.

So go ahead and take a cgroup name and return the actual list
of files under that.  Long as we're doing this, also add an
SCM version and ensure cgroup names are sane.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
xml: add description of Prune

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
cgm: add new calls

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