tzcode: Fix issues when TZ is an absolute path

• If TZ starts with TZDIR, strip any additional slashes so relname does not end up looking like an absolute path. For instance, TZ=/usr/share/zoneinfo//UTC should result in UTC, not /UTC.

• In the setugid case, we were incorrectly passing name rather than relname to fstatat().

• Modify the tz_env and tz_env_setugid test cases to exercise both of these scenarios.

• Also add test cases for invalid values of TZ, which I wrote earlier but forgot to include in a5f14e4f9069.

Reported by: Paul Eggert <eggert@cs.ucla.edu>
MFC after: 3 days
Fixes: 967a49a21a27 ("Update tzcode to 2025b")
Fixes: a5f14e4f9069 ("tzcode: Use -00 only for invalid time zones")
Reviewed by: philip
Differential Revision: https://reviews.freebsd.org/D52753