vcl_cookies: standardize on X-Orig-Cookie
Text was using Orig-Cookie and mobile was using X-Orig-Cookie for
basically identical purposes, so this aligns the two on
X-Orig-Cookie. Note that in the text case (the one changing to
align), the Orig-Cookie header is deleted in beresp for pass/miss
cases, so there's no chance of MW-level code having been exposed
to it or noticing the change.