This has some advantages:
* You get ssh-agent regardless of how you logged in. Previously it was
only started for X11 sessions.
* All sessions of a user share the same agent. So if you added a key
on tty1, it will also be available on tty2.
* Systemd will restart ssh-agent if it dies.
* $SSH_AUTH_SOCK now points to the /run/user/<uid> directory, which is
more secure than /tmp.
For bonus points, we should patch ssh-agent to support socket-based
activation...
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
