tzsetup: add support for countries in two continents

[khorben_defora.org]

Summary

This supports countries located across multiple continents, as per the zone1970.tab file. This only affects Cyprus and Türkiye at the moment.

The patch only handles a single extra continent, even though the file format supports multiple ones; in practice this capability is not used in zone1970.tab as of today. Therefore I believe this change is a net positive for now, as it fixes an issue reported by a user from Cyprus.

PR: #236874
Sponsored by: The FreeBSD Foundation

Test Plan

$ make buildenv
$ cd usr.sbin/tzsetup
$ make
$ /usr/obj/usr/src/usr.sbin/tzsetup/tzsetup
[browse through Asia and Europe, both should list Cyprus and Türkiye]

[emaste]

This looks reasonable overall

usr.sbin/tzsetup/tzsetup.c

279

"extra" continent isn't obvious to me -- maybe "alternate"?

473–475

might be worth a switch now that we have 3 cases

[khorben_defora.org]

This comment was removed by khorben_defora.org.

[khorben_defora.org]

• Renamed the new member of struct country
• Use a switch/case for pass 1 to 3

[emaste]

I fetched this from your branch but it still has struct continent *extra; /* extra continent */ (not alternate), please refresh.

[khorben_defora.org]

I rebased and updated the commit with the full message at https://github.com/khorben/freebsd-src/tree/khorben/tzsetup-extra.