Fix drag-and-drop bugs on Lucid build

Fix drag-and-drop bugs on Lucid build

Also, optimize Fx_translate_coordinates to avoid excessive calls
to _XReply.

* lisp/x-dnd.el (x-dnd-get-drop-rectangle): Return empty drop
rectangle if posn-area.
* src/xfns.c (Fx_translate_coordinates): Accept arg
`require_child'.  If not set, allow optimizations based on
cached position data.
* src/xselect.c (x_handle_dnd_message): Use
x_translate_coordinates.
* src/xterm.c (x_translate_coordinates): Export function.
(x_handle_translate_coordinates): New function.
(handle_one_xevent): Fix coding style.
* src/xterm.h: Update prototypes.