Bug#1068416: ssh-agent: improve systemd user session integration

I have been running with the patches provided by dkg for many months,
and they work great.  I think socket activation is the way to go for the
agent, so I strongly support pulling in the patches to improve the agent
interaction.

I also note that the ssh-agent systemd service currently being provided
by the openssh-client package is broken, and not enablable, and this
patches fixes that.  So all the more reason to pull it in.

Thanks for maintaining!

jamie.