Revert "Remount /sys/fs/cgroup as readonly always"

Adding cgroups path to the Spec.

The only discussion related to this is in [1,2], where the
relationship between oomScoreAdj and disableOOMKiller is raised. But
since 429f936 (Adding cgroups path to the Spec, 2015-09-02, opencontainers#137)
resources has been tied to cgroups, and oomScoreAdj is not about
cgroups.  For example, we currently have (in config-linux.md):

  You can configure a container's cgroups via the resources field of
  the Linux configuration.

I suggested we move the property from linux.resources.oomScoreAdj to
linux.oomScoreAdj so config authors and runtimes don't have to worry
about what cgroupsPath means if the only entry in resources is
oomScoreAdj.  Michael responded with [4]:

  If anything it should probably go on the process

So that's what this commit does.

I've gone with the four-space indents here to keep Pandoc happy (see
7795661 (runtime.md: Fix sub-bullet indentation, 2016-06-08, opencontainers#495),
but have left the existing entries in this list unchanged to reduce
churn.

[1]: opencontainers/runtime-spec#236
[2]: opencontainers/runtime-spec#292
[3]: opencontainers/runtime-spec#137
[4]: opencontainers/runtime-spec#782 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>