tzcode: Add a test case for plain issetugid caseHEADmain

This catches a bug in tzcode which incorrectly considers TZDEFAULT as
unsafe as if it came from the environment.

Also deduplicate some repeated code, and fix a missing mode in an
open(2) call with O_CREAT set.

Event:		Oslo Hackathon 202508
Reviewed by:	philip
Differential Revision:	https://reviews.freebsd.org/D52241

0 files changed, 0 insertions, 0 deletions