Various follow-ups for early init file changes

Various follow-ups for early init file changes

* doc/emacs/custom.texi (Early Init File): Add more details about
which variables must be set in the early init file rather than the
regular init file.  See
https://lists.nongnu.org/archive/html/bug-gnu-emacs/2018-02/msg00827.html

* lisp/emacs-lisp/package.el (package-enable-at-startup): Update
docstring to note that packages are now made available before loading
the init file, rather than afterwards.  See
https://lists.gnu.org/archive/html/emacs-devel/2018-02/msg00632.html
(package-load-list): Refer to "making available" rather than "loading"
for packages.  See
https://lists.gnu.org/archive/html/emacs-devel/2018-02/msg00298.html

* lisp/startup.el (command-line): Call `custom-reevaluate-setting' on
predefined variables before loading the early init file and before
`package-initialize' is called.  This prevents
`Info-default-directory-list' from being unbound when
`package-initialize' tries to access it during startup.  See
https://lists.gnu.org/archive/html/emacs-devel/2018-02/msg00545.html

* lisp/emacs-lisp/package.el (package-initialize): Issue a warning
if called twice.
See: https://lists.gnu.org/archive/html/emacs-devel/2018-02/msg00626.html
     https://lists.gnu.org/archive/html/emacs-devel/2018-03/msg00301.html