tzsetup: adopt zone1970.tab changes

• 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

Reviewed by: bapt, philip
Differential Revision: https://reviews.freebsd.org/D39606