[FIX] Replace API which is not available under 10.6

- Define DISPATCH_TIME_FOREVER as ~0ull if not available.
- Use the global concurrent queue, since private concurrent queues
  are not available on 10.6
Remove duplicate definitions of the sandboxed method.

- GPGTask is responsible for evaluating the sandboxed status
  and all other objects ask GPGTask if they need it.
- sandboxed will always return NO on 10.6, since there seem to be
  problems, where the check can't be run and the application crashes.