ad_return_url: experimental change to produce per default a proper encoded result
Background: before this change, ad_return_url encoded always the query variables but not the URL path. This mixed result is complex to handle to achieve ex-post a proper encoding. The function has a flag "-urlencode", which is nowhere used in OpenACS. Other functions like e.g. export_vars, or pretty_link return per default as well the encoded URL. "ad_return_url" has a new flag "-path_encode=false" to obtain the previous behavior (similar to the other mentioned functions).