Remove pad from struct input_event.

Remove pad from struct input_event.
* termhooks.h (struct input_event): Remove padding field.
Adjust comment.
* keyboard.c (event_to_kboard): Simplify because frame_or_window
member is never cons for a long time.  Adjust comment.
(mark_kboards): Adjust because SELECTION_REQUEST_EVENT and
SELECTION_CLEAR_EVENT has no Lisp_Objects to mark.  Add comment.
* xterm.c (handle_one_xevent): Do not initialize frame_or_window
field of SELECTION_REQUEST_EVENT and SELECTION_CLEAR_EVENT.