Simplify and unify UNIX timezone retrival

[Secrus]

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

Closes: #837
Closes: #840
Closes: #829

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing Secrus:unix-timezone (150aa01) with master (aea611d)}

Summary

✅ 1 untouched benchmark

[Secrus]

@mgorny since this changes how things work for Gentoo, could you check/confirm that /etc/localtime works on Gentoo?

PS if you know any other distro maintainers, please notify/ping them, I would love to get broader info on this matter

[mgorny]

@mgorny since this changes how things work for Gentoo, could you check/confirm that /etc/localtime works on Gentoo?

If you're asking whether tests pass after this change on my system, they do. They also pass if I remove /etc/localtime altogether or copy the specific timezone there. I think that's all the scenarios we "support".

PS if you know any other distro maintainers, please notify/ping them, I would love to get broader info on this matter

CC @hroncok (sorry, don't recall anyone else offhand, though I really should have)