alarm delay => previous-delay, error
delay -- A non-negative integer.
previous-delay -- A non-negative fixnum.
error -- A fixnum.
This function arranges for a SIGALRM signal to be delivered to the calling process in delay seconds.
If delay is zero, any pending alarm is canceled.
In any event any previously set alarm is canceled.
The return value previous-delay is the number of seconds remaining until any previously scheduled alarm was due to be delivered, or zero if there was no previously scheduled alarm.