* lisp/emacs-lisp/package.el: New quickstart feature

* lisp/emacs-lisp/package.el: New quickstart feature

(package--quickstart-pkgs): New var.
(package-activate-1): Obey and fill it.
(package-activate-all): New function.
(package-initialize): Call it.
Set package-initialized before activating the packages.
(package-installed-p): Make it work before package.el is initialized in
the case where min-version is not specified.
(package-install, package-delete): Refresh the quickstart if applicable.
(package-quickstart, package-quickstart-file): New vars.
(package--quickstart-maybe-refresh, package-quickstart-refresh):
New functions.

* lisp/startup.el (command-line): Use package-activate-all rather than
package-initialize.

* doc/lispref/package.texi (Packaging Basics):
* doc/emacs/package.texi (Package Installation):
* doc/lispref/os.texi (Startup Summary): Refer to package-activate-all.