Expose CURLOPT_POSTREDIR

The [`CURLOPT_POSTREDIR` option](https://curl.haxx.se/libcurl/c/CURLOPT_POSTREDIR.html) is e.g. required when the site is redirecting from not-slash-terminated to slash-terminated urls. Currently, the safe wrapper does not have a way to set `CURLOPT_POSTREDIR`, and the curl-sys crate doesn't have constants for `CURL_REDIR_POST_301`, `CURL_REDIR_POST_302` , `CURL_REDIR_POST_303` and `CURL_REDIR_POST_ALL`, which are defined [here](https://github.com/curl/curl/blob/dca6f73613d8b578687bd4aeeedd198f9644bb53/include/curl/curl.h#L2055-L2065).