tzcode: Update to 2026a

[des]

Summary

Many thanks to Paul Eggert for adopting most of our adaptations as
optional features upstream in the previous release (2025c).

MFC after: 1 week

[des]

relax checks when not setugid

[philip]

Thank you for dealing with upstream so patiently. ;-)

It's good to see many of the #ifdef __FreeBSD__ go away.

I've looked closely at the changes in localtime.c in particular and I think this merge looks good. Builds are running now and I'll find time to run some tests.

[des]

reduce diff to upstream

[philip]

I think this looks good. As far as I can tell, these bits pass the test suite (though the tests panic elsewhere, which is tedious). I looked carefully over the localtime.c changes and I don't see anything broken, but I haven't manually tested every FreeBSD-ism.