Disable middle click selection emulation during XDS

Disable middle click selection emulation during XDS

* doc/lispref/frames.texi (Drag and Drop): Document new
variable.
* lisp/x-dnd.el (x-dnd-do-direct-save): Disable drop emulation
during XDS drag-and-drop.

* src/keyboard.c (kbd_buffer_get_event): Handle recorded value.
* src/xterm.c (x_dnd_do_unsupported_drop): Return if new
variable is nil.
(x_dnd_send_unsupported_drop): Record value of new variable in
events.
(x_dnd_begin_drag_and_drop): Handle recorded value.
(syms_of_xterm): New variable `x-dnd-use-unsupported-drop'.