Currently, if arping is found, the script will always exit with exit
code 1. This is because arping sends a ping to cause other devices to
notice the new MAC address, but this ping is not expected to be
returned. However, this means that the exit code from the arping line is
always 1. When running manually this isn't noticeable as it's the last
action performed, but it plays serious havoc when calling pipework from
another program. The fix is to OR the exit code with true.