(read_key_sequence): Check that key is an integer

(read_key_sequence): Check that key is an integer
before comparing it with quit_char.
(add_command_key): Use larger_vector.
(read_char_x_menu_prompt): Instead of converting symbol and
integer events into conses (EVENT . nil), use (EVENT . disabled)
which cannot be confused with legal events.
(read_char): When reading from Vunread_command_events, check for
events of the form (EVENT . disabled) instead of
(EVENT . nil).