This function will convert the given URL encoded input string to a "plain
string" and return that as a new allocated string. All input characters that
are URL encoded (%XX where XX is a two-digit hexadecimal number) will be
converted to their plain text versions.
If the 'length' argument is set to 0, curl_unescape() will use strlen() on the
input 'url' string to find out the size.
You must curl_free() the returned string when you're done with it.
Since 7.15.4, curl_easy_unescape(3) should be used. This function will
be removed in a future release.
A pointer to a zero terminated string or NULL if it failed.