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, or +) will be
-converted to their plain text versions (up to a ? letter, no letters to the
+converted to their plain text versions (up to a ? letter, no + letters to the
right of a ? letter will be converted).
If the 'length' argument is set to 0, curl_unescape() will use strlen() on the