(url-cookie-store): Rename arg PATH to LOCALPART.

(url-cookie-store): Rename arg PATH to LOCALPART.
(url-cookie-retrieve): Likewise.
(url-cookie-generate-header-lines): Likewise.
(url-cookie-handle-set-cookie): Likewise.
(url-cookie-create): Expect :localpart instead of :path.
(url-cookie-localpart): Renamed from url-cookie-path.
(url-cookie-set-localpart): Renamed from url-cookie-set-path.
(url-cookie-file): Doc fix.
(url-cookie-p): Add doc string.