Speed up receiving drops over slow connections

Speed up receiving drops over slow connections

* lisp/x-dnd.el (x-dnd-debug-errors): New variable.
(x-dnd-handle-drag-n-drop-event): Bind
`x-fast-protocol-requests' to t if that is off.

* src/xfns.c (Fx_change_window_property):
(Fx_delete_window_property):
* src/xselect.c (Fx_send_client_message, x_send_client_event):
Don't sync to check for errors if fast protocol requests are
enabled.

* src/xterm.c (x_catch_errors_for_lisp, x_check_errors_for_lisp)
(x_uncatch_errors_for_lisp): New functions.
(syms_of_xterm): New variable `x-fast-protocol-requests'.
* src/xterm.h: Update prototypes.