Add autoclose variables

When passing around fanotify events between threads (e.g., using
Thread::Queue, as I do), autoclosing event fd's results in them
being closed before accessing them in the threads.

The newly added Linux::Fanotify::Event::autoclose variable can be used
to prevent that behavior. A similar variable has been added to the
FanotifyGroup class.

The documentation has been updated to reference the now correct original
man pages as maintained by the linux man page project.