bug#31877: 26.1; Cannot view second certificate information when updating packages

[Allen Li <darkfeline@felesatra.moe>]

1. emacs -Q

Note: This depends on some local state for remembering choices for
certs, so you might want to add
HOME="$(mktemp -d)" to use a clean temporary environment

2. M-: (setq network-security-level 'paranoid) RET
3. M-: (setq package-archives
'(("gnu" . "https://elpa.gnu.org/packages/")
("org" . "https://orgmode.org/elpa/"))) RET

4. M-x package-list-packages RET

5. There should be two windows, one with the package list and one with
certificate information for the ELPA archive.  Emacs prompts for
"Continue connecting? (...)"

6. Press a

7. Now there will be two windows, both displaying the package list.
There will be some success message in the minibuffer.  Emacs is actually
in a state where it is requesting input for the certificate for the
second archive, but this is non obvious because the success message from
the first certificate overrides the prompt for input for the second
certificate [BUG 1].  Also, both windows are displaying the package
list.  The information for the second certificate is not displayed, so
the user cannot check the certificate information to decide whether to
accept it [BUG 2].  Pressing ? displays a help window that explains the
three possible choices, but there is no way to go back to the
certificate information even if it were displayed before, giving the
user no way to verify the certificate information if they accidentally
pressed ? [BUG 3].

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-05-28 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12000000

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 MODULES THREADS LIBSYSTEMD LCMS2