[!MAC_OSX] Include keyboard.h and syssignal.h.

[!MAC_OSX] Include keyboard.h and syssignal.h.
[!MAC_OSX] (target_ticks): Remove variable.
[!MAC_OSX] (check_alarm, pause, index): Remove functions.
[!MAC_OSX && __MRC__] (sys_strftime): Likewise.
[!MAC_OSX] (select): If fd 0 is not set in rfds and some input
event occurs before timeout, behave as if the function were
interrupted.
[!MAC_OSX] (sigblock, sigsetmask, alarm): Simulate SIGALRM
handling using Time Manager routines.
[!MAC_OSX] (mac_atimer_task, mac_atimer_qlink, signal_mask): New
variables.
[!MAC_OSX] (mac_atimer_handler, set_mac_atimer, remove_mac_atimer)
(setitimer): New functions.