Make the URL library use records.

Make the URL library use records.

* lisp/url/url.el, lisp/url/url-cache.el, lisp/url/url-dav.el,
lisp/url/url-expand.el, lisp/url/url-file.el, lisp/url/url-imap.el,
lisp/url/url-ldap.el: Use `url-p' instead of `vectorp'.

* lisp/url/url-http.el (url-http): Check for type `url' instead of
`vector'.