adjust redirect-url option handling more
adjust redirect-url option handling more

don't use the redirect-url option path part as the callback
handler path, use the normal oauth2_proxy prefix for that

follow-up to 1ed776bed2335c249e7529e55049783ca400f09f
inspired by https://github.com/pusher/oauth2_proxy/issues/9#issuecomment-470375518

ploxiln/oauth2_proxy,ploxiln/oauth2_proxy