ftp conn reuse, tighten use_ssl check

[icing]

CURLUSESSL_* needs a bit more special check on
connection reuse. Except CURLUSESSL_TRY, we
expect the reused connection to have exactly
the same setting. On TRY, we are happy with
anything that is no NONE.

[Copilot]

🟡 Changes recommended

The new/changed reuse semantics for CURLUSESSL_* are not backed by targeted test coverage, making regressions in a subtle connection-reuse area more likely.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR tightens FTP-family connection reuse matching around CURLOPT_USE_SSL/use_ssl, ensuring reused connections have compatible TLS-upgrade intent: exact matches for all modes except CURLUSESSL_TRY, which can reuse any connection configured for some SSL use (non-NONE).

Changes:

• Replace the prior use_ssl reuse assertion/ordering check with explicit matching rules.
• Allow CURLUSESSL_TRY transfers to reuse any connection whose use_ssl is not CURLUSESSL_NONE; require exact equality for other modes.

File summaries

File	Description
lib/ftp.c	Updates FTP connection reuse matching logic for use_ssl to be stricter and explicit, with a special-case rule for CURLUSESSL_TRY.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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