Use a periodic itimer instead of repeated calls to alarm() in

Use a periodic itimer instead of repeated calls to alarm() in
sidewaysintpr().  This increases the accuracy of the per-interval
counts when they are interpreted as rates.  Repeated calls to alarm(n)
give an average interval that is about 2 ticks larger than n and has
a large variance.  Periodic itimers normally get the average almost
right but have similarly large variance (due to scheduling delays).

Statistics utilities should use clock_gettime() to determine the
actual interval, but it is still useful to maximize the accuracy of
the interval, especially for cases like netstat -w where counts are
displayed so the program cannot hide the inaccuracy in a rate
conversion.