tzsetup: adopt zone1970.tab changes

[yuripv]

Summary

• assumption that single-zone countries do not have description is no longer correct; do not try to optimize this case as it's only going to make the code more confusing and we now have menus with a single zone selection because of this
• remove the single-country continent short cut, it also only serves to confuse users as we now have such a continent
• instead add a single-zone contry short cut (see above), now all single-zone countries fall here
• use the @# continent overrides that zone1970.tab introduces (this is visible at least fixing Iceland being currently listed under Africa)
• add Arctic Ocean "continent" coming only from the overrides at the moment
• update baseline with the changes

Test Plan

The only really possible testing here is checking that tzsetup can parse zone1970.tab and provided selections still make sense; the latter should be covered by D39634.

[yuripv]

number continent entries consistently with lower-level menus

[yuripv]

grrrr, sorry, really upload the updated patch

[yuripv]

deduplicate added code

[yuripv]

rebase on D39634

[yuripv]

rebase after baseline integration

[bapt]

LGTM, I added @philip as he has been updating the tzdata for a while now, so I might have an opinion

[philip]

Thank you very much!

I gave this a cursory test, and it looks like it works. Thank you for doing this work.