(kbd_buffer_store_event_hold): Don't throw-on-input

(kbd_buffer_store_event_hold): Don't throw-on-input
if first event in [signal xxx] sequence.
(lispy_user_signals, Qusr1_signal, Qusr2_signal): Remove.
(syms_of_keyboard): Don't intern and staticpro them.
(Qsignal, Qusr1, Qusr2): Declare.
(syms_of_keyboard): Intern and staticpro them.
(make_lispy_event): Return them for USER_SIGNAL_EVENTs.