bug#35414: 26.2; ELPA packages signed with second, unknown key

[Stefan Monnier]

> No, the bug is that the signature verification should not signal an
> error before September 2019 even if you don't have the new key.
>
> Could you remove the gnu-elpa-keyring-update package, and the 2019
> key from your keyring and try and help us figure out why you get
> those errors and I don't?

Oh, wait, I see it now: I had set package-check-signature incorrectly.
So, I can reproduce the problem now with

    (setq package-check-signature t)
    
It works correctly if you've set it to the default `allow-unsigned`.

I think it's a mistake: `allow-unsigned` should mean to allow installing
packages when they don't have a signature at all, and `t` should mean
to allow installing if at least one of the sigs is verified rather than
only if all the sigs are verified.

But that ship has sailed, so I'm going to have to rethink the transition
to the new key.  Damn!


        Stefan