tzcode: Resurrect tzsetwall(3) with a deprecation warning.

This function has been around since 4.4BSD but was dropped upstream in 2020. This went unnoticed when tzcode was updated. Bring it back, but prepare for removing it before 14.0 is released.

PR: 269445
MFC after: 3 days
Reported by: val@packett.cool
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D38445
Approved by: re (cperciva)

(cherry picked from commit 71e0c8906e1a4ff774f8ae810aeb94af039eedb7)
(cherry picked from commit eab9ed3d48e0842f3114d8582a4c2868a354190b)