(cmd_error): Use clear_prefix_arg.

(cmd_error): Use clear_prefix_arg.
(internal_last_event_frame, Vlast_event_frame): Normal vars again.
All uses changed.
(Quniversal_argument, Qdigit_argument, Qnegative_argument): Declare.
(clear_prefix_arg, finalize_prefix_arg, describe_prefix_arg): New fns.
(command_loop_1): Handle digits and minus specially, when they're
part of a prefix arg.
Handle universal-argument and digit-argument and negative-argument
bindings here, rather than doing I/O in the Lisp code.
(read_char): When reading switch-frame events from the side queue,
set internal_last_event_frame.
(readable_events): Return non-zero if a side queue has data.
(kbd_buffer_get_event): Don't abort if event has no associated frame.
(read_key_sequence): Improve behavior when there's no current display.
(init_perdisplay): Initialize the new members.