[tz] Patch for ziguard.awk

I discovered that the ziguard.awk is not entirely reversible: it doesn't switch between vanguard and rearguard data for Namibia data completely in all cases. Attached is a patch with some more explanation and a proposed fix. It's bigger than strictly necessary because I tidied up the Ireland lines rather than applying the same fix there. Low severity / impact and probably not an issue in the normal release workflow : I discovered it because I was stumbling around generating my own tarballs for testing the proposed 2020b changes. That's not something I usually do. As always, thanks for all the work you do. Neil. -- Google UK Limited Registered Office: 6 Pancras Square, London, N1C 4AG Registered in England Number: 3977902