[RDY] Reimplement :drop

The :drop ex command was inadvertently removed when removing FEAT_GUI. See issue #1132.
This patch reintroduces it.
vim had the following:

    /*
     * Redefine the argument list.
     */
    void set_arglist(char_u *str)
    {
        do_arglist(str, AL_SET, 0);
    }

This one-line function was called exactly once in the codebase, which is a pretty bad code smell IMO. I inlined it while applying the patch.