(kbd_buffer_store_event): Fix interrupt_signal prototype.

(kbd_buffer_store_event): Fix interrupt_signal prototype.
Pass a dummy argument when calling interrupt_signal.
(parse_menu_item): Mark disabled items before checking for empty def.
(read_char_minibuf_menu_prompt): Make safety more visible.
(read_key_sequence): Add a `first_unbound' variable.
Use it to detect C-c ESC ESC ESC ESC ... cases and drop the
unbound prefix as soon as we can detect it.