Start ssh-agent as a user unit

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...

nixos/modules/config/gnu.nix

···
     # GNU lsh.
     services.openssh.enable = false;
     services.lshd.enable = true;
-    services.xserver.startOpenSSHAgent = false;
+    programs.ssh.startAgent = false;
     services.xserver.startGnuPGAgent = true;
 
     # TODO: GNU dico.

nixos/modules/programs/ssh.nix

···
           for help.
         '';
       };
+
+      startAgent = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether to start the OpenSSH agent when you log in.  The OpenSSH agent
+          remembers private keys for you so that you don't have to type in
+          passphrases every time you make an SSH connection.  Use
+          <command>ssh-add</command> to add a key to the agent.
+        '';
+      };
+
     };
+
   };
 
   config = {
···
           target = "ssh/ssh_config";
         }
       ];
+
+    # FIXME: this should really be socket-activated for über-awesomeness.
+    systemd.user.services.ssh-agent =
+      { enable = cfg.startAgent;
+        description = "SSH Agent";
+        wantedBy = [ "default.target" ];
+        serviceConfig =
+          { ExecStart = "${pkgs.openssh}/bin/ssh-agent -a %t/ssh-agent";
+            Type = "forking";
+            Restart = "on-failure";
+          };
+      };
+
+    environment.extraInit = optionalString cfg.startAgent
+      ''
+        if [ -z "$SSH_AUTH_SOCK" -a -n "$XDG_RUNTIME_DIR" ]; then
+          export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/ssh-agent"
+        fi
+      '';
+
   };
 }

nixos/modules/rename.nix

···
 ++ obsolete [ "services" "sshd" "gatewayPorts" ] [ "services" "openssh" "gatewayPorts" ]
 ++ obsolete [ "services" "sshd" "permitRootLogin" ] [ "services" "openssh" "permitRootLogin" ]
 ++ obsolete [ "services" "xserver" "startSSHAgent" ] [ "services" "xserver" "startOpenSSHAgent" ]
+++ obsolete [ "services" "xserver" "startOpenSSHAgent" ] [ "programs" "ssh" "startAgent" ]
 ++ obsolete [ "services" "xserver" "windowManager" "xbmc" ] [ "services" "xserver" "desktopManager" "xbmc" ]
 
 # KDE

nixos/modules/services/x11/display-managers/default.nix

···
 
       ''}
 
-      ${optionalString cfg.startOpenSSHAgent ''
-        if test -z "$SSH_AUTH_SOCK"; then
-            # Restart this script as a child of the SSH agent.  (It is
-            # also possible to start the agent as a child that prints
-            # the required environment variabled on stdout, but in
-            # that mode ssh-agent is not terminated when we log out.)
-            export SSH_ASKPASS=${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass
-            exec ${pkgs.openssh}/bin/ssh-agent "$0" "$sessionType"
-        fi
-      ''}
-
       ${optionalString cfg.startGnuPGAgent ''
         if test -z "$SSH_AUTH_SOCK"; then
             # Restart this script as a child of the GnuPG agent.

nixos/modules/services/x11/xserver.nix

···
         '';
       };
 
-      startOpenSSHAgent = mkOption {
-        type = types.bool;
-        default = true;
-        description = ''
-          Whether to start the OpenSSH agent when you log in.  The OpenSSH agent
-          remembers private keys for you so that you don't have to type in
-          passphrases every time you make an SSH connection.  Use
-          <command>ssh-add</command> to add a key to the agent.
-        '';
-      };
-
       startGnuPGAgent = mkOption {
         type = types.bool;
         default = false;
···
     hardware.opengl.videoDrivers = mkIf (cfg.videoDriver != null) [ cfg.videoDriver ];
 
     assertions =
-      [ { assertion = !(cfg.startOpenSSHAgent && cfg.startGnuPGAgent);
+      [ { assertion = !(config.programs.ssh.startAgent && cfg.startGnuPGAgent);
           message =
             ''
-              The OpenSSH agent and GnuPG agent cannot be started both.
-              Choose between `startOpenSSHAgent' and `startGnuPGAgent'.
+              The OpenSSH agent and GnuPG agent cannot be started both. Please
+              choose between ‘programs.ssh.startAgent’ and ‘services.xserver.startGnuPGAgent’.
             '';
         }
         { assertion = config.security.polkit.enable;