Cygwin: timerfd: rework implementation

timerfd_tracker and timerfd_shared classes:

- Just because handles are shared, we don't have to store them in
  shared memory.  Move share handles into timerfd_tracker class.

- Drop shared instance counter since it's not required anymore.
  timerfd_shared only stores the actual timer data.

- Drop timerfd_shared::create, just set clock id.

- Drop timerfd_shared::dtor, it's not required anymore.

- Drop timerfd_tracker::close, just call dtor where required.

- Rename timerfd_tracker::increment_instances to timerfd_tracker::dup.
  It's the only reason it exists...

- timerfd_tracker::dtor now checks the non-shared pointers for NULL
  before attempting to close them.

- timerfd_tracker::dtor handles decrementing the local instance count
  by itself.

- Add a method timerfd_tracker::init_fixup_after_fork_exec to set
  non-shared pointers to NULL.  Together with the dtor patches it
  fixes a problem with close_on_exec timerfd descriptors.

- Fix a bug in handling the thread synchronization event.  It's
  actually nice to create it before using it...

- Drop using sec_none{_nih} in InitializeObjectAttributes.  It's
  an unnecessary roundabout route just to get a NULL pointer.

- Slightly rework timechange window handling.

- Add more comments to explain what happens.

fhandler_timerfd:

- Drop cnew macro, it just hides what happens.

- fhandler_timerfd::fixup_after_exec now calls
  timerfd_tracker::init_fixup_after_fork_exec first, so a subsequent
  call to timerfd_tracker::dtor only works on valid handles.

- fhandler_timerfd::close directly calls timerfd_tracker::dtor now.

- Drop dtor call in fhandler_timerfd destructor.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.h

@@ -2682,7 +2682,7 @@ class fhandler_timerfd : public fhandler_base
  public:
   fhandler_timerfd ();
   fhandler_timerfd (void *) {}
-  ~fhandler_timerfd ();
+  ~fhandler_timerfd () {}
 
   fhandler_timerfd *is_timerfd () { return this; }