sleeps for approximately t seconds. Returns a true value; if the
return value is numeric, it will be the actual duration of the sleep.
If the second argument is a true value, the function may return
earlier than the indicated time interval as the result of a signal
being received.
If this function fails, it returns nil plus a string describing the
error. In particular, os.delay(0, true) will return an error if the
operating system is not capable of interrupting delays, and
os.delay(0) will return an error if the os library does not
implement delay.
