Allow calls to fetch_url to specify a cookiejar file

This one might be controversial.

My case is my retriever plugin, which sometimes wants to retrieve content from sites that refuse to hand it over without setting a cookie.  This change adds the option of specifying a cookiejar file to hold the cookies.  What happens to the cookiejar file afterwards is for the caller to decide.

There are quite a lot of arguments to fetch_url, it probably should change to named arguments.