Add test_31_13 to check connection reuse on mixed --ssl-reqd setting.
For that add debug env var CURL_DBG_NO_USE_SSL_ON_FIRST to disable
--ssl-reqd for the first url. Check that the connection without SSL
from the first url is not reused on the second URL that requires it.
Tweak special ftp: protocol check to fail a DEBUGASSERT on mismatched
`use_ssl` settings as that should have been caught before in the
connection reuse matching (imap/smtp etc. do not have this extra check
and rely on the general part doing its job).
Closes#21665
