This is a first draft; probably needs some more cleanup.
- Install zone1970.tab in addition to zone.tab.
- The old logic to remove countries from zone.tab doesn't port to zone1970.tab since some lines might have to be patched more than once. Replace it with some awk.
- remove LEAPFILE reference that wasn't being set anyway
- Fix plist entry for Etc/UCT, which is (since 2019a) part of "backward" and not present otherwise
- Make BACKWARD not be the default, since (a) it's not the base system's default and (b) that stuff is all deprecated
- Some misc. tweaks