49: Cookie-jar save transfers group access to a different GID

`Curl_fopen()` copies the old cookie jar's permission bits onto a newly created temp file, but the temp file may receive a different GID (the process's primary group instead of the old file's group). After rename, group-permission bits that were intended for one group now authorize a different …

if(sb.st_uid != geteuid())
  sb.st_mode = 0;
/* ... */
fd = curlx_open(tempstore, O_WRONLY | O_CREAT | O_EXCL,
                S_IRUSR | S_IWUSR | sb.st_mode);