bug#39168: 27.0.50; Error on command line flags with use dump-file flag with other flags

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Fri, 17 Jan 2020 23:26:48 -0500
> 
> 
> - dump an image: emacs --batch -q --eval '(dump-emacs-portable
>   "~/.emacs.d/emacs.pdmp")'
> - start emacs: ~/bin/emacs -q --dump-file=/Users/yuan/.emacs.d/emacs.pdmp -g 
> 151x50
> - error: command-line-1: Unknown option 
> ‘--dump-file=/Users/yuan/.emacs.d/emacs.pdmp’

Not _any_ other flag, but flags parsed in command-line-1.

If we restored Emacs from a dump file, we should filter --dump-file
(and the following argument, if --dump-file doesn't end in a '=') in
init_cmdargs, so that startup.el doesn't see it.