Fix url-copy-file argument handling

Fix url-copy-file argument handling

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-05/msg00500.html
* lisp/url/url-handlers.el: Update autoloaded docstrings.
Quote function symbols as such.
(url-handler-regexp): Make grouping construct shy.
(url-file-handler, url-insert-buffer-contents)
(url-handlers-create-wrapper, url-handlers-set-buffer-mode):
Simplify.
(url-file-handler-identity): Clarify calling convention.
(file-name-absolute-p, url-file-local-copy): Mark ignored arguments
as such.
(url-handler-directory-file-name): Prefer string comparison over
regexp match where either will do.
(url-copy-file): Handle integer as third argument as per copy-file.