[JIRA] (BIT-1407) -f silently fails if base/frameworks/packet-filter isn't loaded

If we had that, the '-f' switch would probably be added by the packet
filter script, so it'd be available only when loaded.

I'd like to move argument processing into script-land as well.
Generally, however, I think it would still be good to avoid arguments
controlling script behaviour as much as possible. Main reason is
argument inflation: -f has been historically around but there are
plenty other scripts that in principle could take command-line
arguments as well, which would get messy. And we have the X=Y syntax
already to take care of that.