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

tzsetup: add baseline file to control parsed zonetab contents

Introduce undocumented option -d to dump parsed zonetab file contents
and add a "baseline" target along with instructions on updating it.

Reviewed by: philip
Differential Revision: https://reviews.freebsd.org/D39634