(kbd_buffer_store_event_hold): New function to store

(kbd_buffer_store_event_hold): New function to store
an event into kbd fifo, but with special handling of quit event;
a quit event is saved for later, and further events are discarded
until the saved quit event has been processed.
(kbd_buffer_store_event): Use kbd_buffer_store_event_hold.
(gen_help_event): Store help event in kbd fifo.
(NREAD_INPUT_EVENTS): Remove.
(read_avail_input): 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).