(BUFFER_SIZE_FACTOR): Remove.

(BUFFER_SIZE_FACTOR): Remove.
(read_input_waiting): Adapt to new read_socket_hook interface.
Remove allocation and initialization of local input_event buffer,
as read_socket_hook stores events directly in fifo.  Allocate and
initialize local hold_quit event to handle postponed quit event
(and store it if set by kbd_buffer_store_event_hold).