curl: add --ca-native and --proxy-ca-native

These are two boolean options to ask curl to use the native OS's CA
store when verifying TLS servers. For peers and for proxies
respectively.

They currently only have an effect for curl on Windows when built to use
OpenSSL for TLS.

Closes #11049

src/tool_operate.c

@@ -1779,7 +1779,9 @@ static CURLcode single_transfer(struct GlobalConfig *global,
               (config->proxy_ssl_allow_beast ?
                CURLSSLOPT_ALLOW_BEAST : 0) |
               (config->proxy_ssl_auto_client_cert ?
-               CURLSSLOPT_AUTO_CLIENT_CERT : 0);
+               CURLSSLOPT_AUTO_CLIENT_CERT : 0) |
+              (config->proxy_native_ca_store ?
+               CURLSSLOPT_NATIVE_CA : 0);
 
             if(mask)
               my_setopt_bitmask(curl, CURLOPT_PROXY_SSL_OPTIONS, mask);