Make X error checking more asynchronous

Make X error checking more asynchronous

This makes some operations (such as x-get-atom-name and
x-change-window-property) up to 2600x faster by avoiding calls
to XSync when setting up an error handler, and when checking for
or uncatching errors if all requests were already processed.

* src/xterm.c (X_COMPARE_SERIALS): New macro.
(struct x_error_message_stack): Add new field `first_request'.
(x_find_error_handler): New function.
(x_error_catcher): New parameter `stack'.  Use it instead.
(x_catch_errors_with_handler): Keep a record of the next
protocol request serial inside the stack entry.
(x_uncatch_errors, x_check_errors, x_had_errors_p, x_clear_errors)
(x_error_handler): Avoid XSync if all requests were processed by
the X server and look for the error handler matching the display
and request serial when processing errors.