parsedate: bsearch the time zones

[bagder]

There are 69 entries, bsearch is faster than linear search for random access.

This now also makes the matching case sensitive (zone names always in uppercase). No docs said otherwise and all tests assumed uppercase.

[Copilot]

Pull request overview

Improve parsedate timezone parsing performance by switching the timezone-name lookup from linear scan to bsearch() over an alphabetically sorted table, while making timezone-name matching case-sensitive.

Changes:

• Alphabetize the static timezone table to support binary search.
• Add a tzcompare() comparator and use bsearch() for timezone lookups.
• Change tzinfo.offset storage to int16_t and add curlx_strcopy() usage for building the search key.

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[bagder]

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[augmentcode]

🤖 Augment PR Summary

Summary: This PR optimizes timezone parsing in lib/parsedate.c by switching from a linear scan to a binary search over the known timezone table.

Changes:

• Reworks the static timezone table into an ASCII/lexicographically sorted list suitable for bsearch().
• Adds a tzcompare() comparator and updates checktz() to build a fixed-size search key and perform the binary search.
• Updates struct tzinfo to store offsets in int16_t (minutes) to reduce table footprint.
• Introduces a dependency on curlx_strcopy() to safely copy non-NUL-terminated input substrings into the search key.
• Makes timezone matching case-sensitive (zone names are defined in uppercase), aligning behavior with existing assumptions.
• Extends tests/libtest/lib517.c with new cases to assert that lowercase zone names (e.g., cet, utc, gmt) are rejected.

Technical Notes: Correctness depends on keeping the timezone list sorted according to strcmp(), since the comparator is now case-sensitive and order-sensitive.

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[augmentcode]

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