Ignore mouse movement correctly on Haiku during drag and drop

Ignore mouse movement correctly on Haiku during drag and drop

* src/haikuselect.c (haiku_unwind_drag_message): New function.
(Fhaiku_drag_message): Set `haiku_dnd_in_progress' to false.
* src/haikuterm.c (haiku_read_socket): Fix overriding of
need_flush when reading events from multiple frames.
* src/haikuterm.h (haiku_dnd_in_progress): New variable.
* src/xdisp.c (note_mouse_highlight): Ignore if said variable is
true.