rename ftp_ssl: the struct field is used for many protocols

Now called 'use_ssl' instead, which better matches the current CURLOPT
name and since the option is used for all pingpong protocols (at least)
it makes sense to not use 'ftp' in the name.

lib/url.c

@@ -2169,7 +2169,7 @@ CURLcode Curl_setopt(struct SessionHandle *data, CURLoption option,
     /*
      * Make transfers attempt to use SSL/TLS.
      */
-    data->set.ftp_ssl = (curl_usessl)va_arg(param, long);
+    data->set.use_ssl = (curl_usessl)va_arg(param, long);
     break;
 #endif
   case CURLOPT_FTPSSLAUTH: