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.

build/jam/images/definitions/minimum

@@ -41,6 +41,7 @@ SYSTEM_APPS = [ FFilterByBuildFeatures
 	CharacterMap
 	Debugger DeskCalc Devices DiskProbe DiskUsage DriveSetup
 	Expander
+	Login
 	NetworkStatus
 	ProcessController
 	ShowImage StyledEdit
@@ -277,6 +278,10 @@ SEARCH on $(networkSettingsFiles)
 	= [ FDirName $(HAIKU_TOP) data settings network ] ;
 AddFilesToHaikuImage system settings network : $(networkSettingsFiles) ;
 
+local userLaunchScripts = <data!launch>user ;
+SEARCH on $(userLaunchScripts) = [ FDirName $(HAIKU_TOP) data launch ] ;
+AddFilesToHaikuImage home config non-packaged data launch : $(userLaunchScripts) ;
+
 # fresh install indicator file for the post install scripts
 SEARCH on <post-install>fresh_install
 	= [ FDirName $(HAIKU_TOP) data system settings ] ;