launch_daemon: Improved target support.

* You can now put jobs/services into a target.
* Instead of having Login started as part of the normal boot process,
it's now in the "login" target.
* The app_server now launches the login target when a login becomes
available (ie. during startup, but that could be improved later on).

launch_daemon: Basic user session implementation.

* Instead of launching Tracker/Deskbar directly, we now launch the
Login application.
* This will now start a new session for the selected user (the password
is currently ignored).
* When a user session is started, the launch_daemon forks, and the
child then restarts the LaunchDaemon application in user mode.
* It then registers itself with its parent, in order to resolve user
dependent services.
* Added a user launch file that will cause Tracker, and Deskbar to
start in the new session.

launch_daemon: Improved target support.

* You can now put jobs/services into a target.
* Instead of having Login started as part of the normal boot process,
it's now in the "login" target.
* The app_server now launches the login target when a login becomes
available (ie. during startup, but that could be improved later on).

launch_daemon: Basic user session implementation.

* Instead of launching Tracker/Deskbar directly, we now launch the
Login application.
* This will now start a new session for the selected user (the password
is currently ignored).
* When a user session is started, the launch_daemon forks, and the
child then restarts the LaunchDaemon application in user mode.
* It then registers itself with its parent, in order to resolve user
dependent services.
* Added a user launch file that will cause Tracker, and Deskbar to
start in the new session.