cgroups: support cgroups mounted in multiple places (v3)

cgroups: support cgroups mounted in multiple places (v3)

(sorry for the extra traffic.)

With this patch, lxc works for me both with all cgroups mounted with
ns cgroup on /cgroup, and with libcgroup mounting all cgroups
separately.

To do this, instead of looking for one cgroup called 'lxc' or
otherwise taking the first cgroup we find, we actually create a
container in every mounted cgroup fs.  Right now it's done under the
root of each fs.  We may want to put that under lxc, or, better yet,
make that configurable.

Changelog:
  Michael H. Warfield: Handle the case where subsystem doesn't have '.'.
  Daniel Lezcano: clean up incorrect reentrant use of mntent helpers
  v3: use the rest of Daniel's cleanups

TODO: add a configurable directory name, 'lxc' by default, under which
      all lxc cgroups are created (i.e. /sys/fs/cgroup/lxc)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Tested-by: Michael H. Warfield <mhw@WittsEnd.com>