Improve the shell cgroup feature

Improve the shell cgroup feature

* Make the cgroup name configurable, so that it can be switched on and
  off in configuration, or so a different cgroup can be used for
  different wikis. The DefaultSettings.php entry also makes the feature
  more discoverable and so more useful to non-Wikimedia users.

* Send named parameters to the shell script instead of numbered
  parameters. Send the command as argument 1, for easier ps -f
  interpretation.

* Move the wrapper script to includes/, since it doesn't need to be
  executable anymore, so it doesn't make sense for it to be in bin/.

* Remove the requirement for a release script to be used. Clean up in
  the wrapper script instead. The release script had to be configured
  globally, which might not be suitable for all MediaWiki installations.

Change-Id: Ic8362cab63df162852d5df56cf695ef163cd6232