Passthrough args looks for the an instance of -- and re-slices the arg
list to pass through those args to the underlying connect exec command.
However such commands may also need to parse their own '--', such as
kubectl exec.
This change updates the logic so that boundary arg parser only considers
the first instance of '--' when deciding which args to pass to the
-exec= commands.
Fixes: #1857
Justen Walker
8279397dbb