cgfs: make sure we use valid cgroup mountpoints

cgfs: make sure we use valid cgroup mountpoints

If lxcfs starts before cgroup-lite, then the first cgroup mountpoints in
/proc/self/mountinfo are /run/lxcfs/*.  Unprivileged users cannot access
these.  So privileged containers are ok, and unprivileged containers are ok
since they won't cache those to begin with.  But unprivileged root-owned
containers cache /run/lxcfs/* and then try to use them.

So when doing cgroup automounting check whether the mountpoints we have
stored are accessible, and if not look for a new one to use.

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