conn: fix connection reuse when SSL is optional

conn: fix connection reuse when SSL is optional

In curl 8.12 I tried to improve the logic on how we handle connections
that "upgrade" to TLS later, e.g. with a STARTTLS. I found the existing
code hard to read in this regard. But of course, the "improvements" blew
up in my face.

We fixed issues with imap, opo3, smtp in 8.12.1, but ftp was no longer
reusing existing, upgraded control connections as before. This PR adds
checks in our pytest FTP tests that verify reuse is happening as
intended.

I rewrote the logic in url.c again, so that the new test checks now pass.

Reported-by: Zenju on github
Fixes #16384
Closes #16392