gnupg2 gpg-agent instead of gnome-keyring

[Stef Walter <stefw gnome org>]

gnome-keyring will no longer contain a gpg-agent in the future. GnuPG
treats its agent as an integral part of gpg2, and shouldn't be
reimplemented in various places.

This unbreaks use of gnupg2 with GNOME, as well as some issues with
using smart cards with gpg.

Neal and also Yuuma have done good work integrating gnome-shell
prompting and libsecret into the gnupg2 pinentry program. So you'll
still see similar prompts, and be able to cache passphrases if desired.

The gpg-agent is started automatically by gpg2 on demand.

One issue with this that it looks like only gpg2 is supported by this
new setup. The changes don't work with the older GnuPG 1.4.x series.

Releases required for the new setup:

 * gnome-keyring without the gpg-agent 3.17.4
 * pinentry 0.9.5
 * gnupg 2.1.6

The distribution that ships GNOME and gnupg2 should setup
/usr/bin/pinentry-gnome3 as the default pinentry program.

Cheers,

Stef