Add Bandwidth File KeyValues to show the XOFF stream events.

As @mikeperry commented by IRC.

Should we show them per RelayLine?, should we show the number of times it was received (in a similar way to the number of errors? or just that it was received?

Since XOFF might depend on the scanner machine or Web server, should we rather show the number of events received in the Bandwidth File Header?. We don't show the URL of the Web servers that were used for the measurements, so in the case the bwauth uses several we wouldn't know which one is the responsible.

For this change we should also create an issue to change the bandwidth-file-spec.txt.

Should we implement this just for onbasca?