SLING-2791 add support to prevent shutdown hook installation

- New command line option -n to not install shutdown hook
- New system property sling.shutdown.hook to control
  shutdown hook installation
- New property sling.shutdown.hook to Main constructor
  to control shutdown hook installation (constructor
  property overwrites system property)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1456861 13f79535-47bb-0310-9956-ffa450edef68