This function will convert the given input string to an URL encoded string and
return that as a new allocated string. All input characters that are not a-z,
A-Z or 0-9 will be converted to their "URL escaped" version (%NN where NN is a
two-digit hexadecimal number).
If the 'length' argument is set to 0, curl_escape() 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_escape(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.