Make sure we use a pinentry without regressions on Stretch
The ISO ships both pinentry-gnome3 and pinentry-gtk2. Let's make sure that we use one that does not introduce regressions compared to what we have in Tails 2.x. And then drop the other.
Explicitly select pinentry-gtk2 as our preferred pinentry program.
On Stretch, gnome-keyring depends on pinentry-gnome3, and then that one
is selected by default. It does not feel worth it to introduce a hackish
solution such as a fake pinentry-gnome3 package, so let's ignore it and
just make sure we are using the pinentry program we prefer (pinentry-gtk2).