curl: CURLOPT_PROXY_CRLFILE / CURLOPT_PROXY_ISSUERCERT / CURLOPT_PROXY_ISSUERCERT_BLOB silently ignored on backends that don't support them

From the Mythos report 2026-05-06 F1. CURLOPTPROXYCRLFILE / CURLOPTPROXYISSUERCERT / CURLOPTPROXYISSUERCERTBLOB silently ignored on backends that don't support them — severity Low https://github.com/curl/curl/blob/455bebc2c7/lib/setopt.cL1786-L1797...