ssh-agent: Use stock ssh-agent

This patch removes our own implementation of ssh-agent and switches to
using the ssh-agent program provided by OpenSSH.  We can't simply drop
the ssh-agent functionality from gnome-keyring, as it enables the
following:

 * Automatic loading and unlocking of keys
 * Prompting in the UI

Instead we wrap the ssh-agent program as a subprocess and augment
the protocol as we need.

Co-authored-by: Daiki Ueno <dueno@src.gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=775981