Correct and simplify timeouts_readd()

William Ahern tells me that the intent here is that timeout_readd()
should always reschedule the timeout at the first time in the future
that is an even multiple of to->interval, as if we had called:

         do {
                 to->expires += to->interval;
         } while (to->expires <= T->curtime);

But of course, that's not efficient.  The implementation strategy
used in this patch simplifies the calculation down to a single %
operation, plus a few additions and subtractions.

To verify the correctness of the formula used here, note first that
        0 <= r < to->interval, and so
        0 < to->interval - r <= to->interval.  Since
        expires' = curtime + (interval - r),
        curtime < expires' <= curtime + interval,
and so the new expiration time is no more than one interval after
curtime.

Note second that since
        r = (curtime - expires) % interval,
        expires' = curtime + (interval - r), we have
        (expires' - expires) % interval =
            (curtime + (interval - r) - expires) % interval =
            (curtime - r - expires) % interval =
            (curtime - (curtime-expires) % interval - expires) % interval =
            (curtime - curtime + expires - expires) % interval =
            0.
And so the new expiration time is an even multiple of interval from
the original expiration time.

Since we have both properties we wanted, this formula should be right.

timeout.c

@@ -358,18 +358,13 @@ static void timeouts_readd(struct timeouts *T, struct timeout *to) {
 	to->expires += to->interval;
 
 	if (to->expires <= T->curtime) {
-		if (to->expires < T->curtime) {
-			timeout_t n = T->curtime - to->expires;
-			timeout_t q = n / to->interval;
-			timeout_t r = n % to->interval;
-
-			if (r)
-				to->expires += (to->interval * q) + (to->interval - r);
-			else
-				to->expires += (to->interval * q);
-		} else {
-			to->expires += to->interval;
-		}
+		/* If we've missed the next firing of this timeout, reschedule
+		 * it to occur at the next multiple of its interval after
+		 * the last time that it fired.
+		 */
+		timeout_t n = T->curtime - to->expires;
+		timeout_t r = n % to->interval;
+		to->expires = T->curtime + (to->interval - r);
 	}
 
 	timeouts_sched(T, to, to->expires);