Allow specifying whether to inhibit cookies on a per-URL basis

Allow specifying whether to inhibit cookies on a per-URL basis

* url-http.el (url-http-create-request): Don't send cookies unless
requested.
(url-http-parse-headers): Don't store cookies unless requested.

* url.el (url-retrieve): Ditto

* url-queue.el (url-queue-retrieve): Take an optional
`inhibit-cookies' parameter.

* url-parse.el (url): Add the `use-cookies' slot to the URL struct
to be able to keep track of whether to do cookies or not on a
per-URL basis.