Add an "mark upgradable packages" command to Package Menu mode.

Add an "mark upgradable packages" command to Package Menu mode.

* lisp/emacs-lisp/package.el (package-alist): Fix risky-local-variable
declaration.
(package--add-to-archive-contents): If there is a duplicate entry
with an older version, remove it.
(package-menu-mark-delete, package-menu-mark-install)
(package-menu-mark-unmark): Make unused args optional.
(package-menu-mark-obsolete-for-deletion): Use
package-menu-get-status instead of a regexp search.
(package-menu-get-status): Use tabulated-list-entry.
(package-menu-mark-upgrades): New command.
(package-menu-mode-map): Bind it to U.
(package-menu-execute): Do installation before deletion.
(package-menu-refresh, package-menu-execute): Use derived-mode-p
instead of checking major-mode.
(package-menu--find-upgrades): New function.