Fixes for Ctrl-G support on carbon, replacing old timeout based polling

Fixes for Ctrl-G support on carbon, replacing old timeout based polling
with alarm based polling.
mac.c (sys_select): Redo sys_select to use alarm-based
polling instead of 1 sec timeouts (like solaris).
macterm.c (x_make_frame_visible): Comment in polling on
frame creation.
keyboard.c: Undef SIGIO on Carbon
atimer.c (alarm_signal_handler): Call alarm handlers after
scheduling.
eval.c (Feval): Remove quit_char test
process.c (wait_reading_process_input): Remove clearing
stdin for select call on process input