mwan3: disable DNS lookups for ping checks

mwan3: disable DNS lookups for ping checks

By default, ping does a reverse DNS of the IP that you are pinging.
When you have a network issue (such as when a link has just gone down
and you haven't yet marked it down), this lookup can cause failures on
tests for links that are still good.

This option only works for iputils ping.
For busybox the option is not evaluated, but it is accepted without
throwing an error.

Fixes: #14968
Fixes: #14924
Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Suggested-by: David Lang <david@lang.hm>