tzcode: Update to 2026a

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

MFC after: 1 week
Reviewed by: philip
Differential Revision: https://reviews.freebsd.org/D55741

(cherry picked from commit ff2c98b30b57b9763e2a6575f729bab676e6c025)