Update FindCgroupMountpoint to be more forgiving

On Gentoo, the memory cgroup is mounted at /sys/fs/cgroup/memory, but the mount line looks like the following:
memory on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)

(note that the first word on the line is "memory", not "cgroup", but the other essentials are there, namely the type of cgroup and the memory mount option)
Merge pull request #464 from tianon/patch-1

- Runtime: adapt cgroup capability detection to work on Gentoo