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