Use /proc/mounts instead of mount(8) to find cgroup mountpoints

Specifically, Ubuntu Precise's cgroup-lite script uses mount -n
to mount the cgroup filesystems so they don't appear in mtab, so
detection always fails unless the admin updates mtab with /proc/mounts.

/proc/mounts is valid on just about every Linux machine in existence and
as a bonus is much easier to parse.

I also removed the regex in favor of a more accurate parser that should
also support monolitic cgroup mounts (e.g. mount -t cgroup none /cgroup).