PR bin/36997 Zafer Aydogan: ping doesn't validate numeric inputs enough.

Reject packet intervals < 1 ns as they lead to infinite loops adding
zero timespecs.

Fix the behind-schedule behavior so it doesn't spend all its time in
that loop adding very small timespecs. Try ping -c 500 -i 0.000000001
to see this in action with the old ping.

(dholland)