Correct another unexpected error caused by set -e
o Using short-circuit (( <variable> )) && ... will evaluate falsey if <variable>
  contains nothing, which is useful; however, this is considered an "error"
  by set -e, causing the program to exit.
o Add --trace|-x option to enable bash set -x command tracing
Sort out and fix issues with gratuitous arping
o Make optional, to avoid issue when not available in the container; we
  cannot check for availability from outside!
o Note what version of arping is required; there are two (incompatible)