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.

headers/private/app/LaunchDaemonDefs.h

@@ -24,7 +24,9 @@ namespace BPrivate {
 
 // message constants
 enum {
-	B_GET_LAUNCH_DATA			= 'lncd',
+	B_GET_LAUNCH_DATA			= 'lnda',
+	B_LAUNCH_SESSION			= 'lnse',
+	B_REGISTER_LAUNCH_SESSION	= 'lnrs',
 };
 
 

headers/private/app/LaunchRoster.h

@@ -22,7 +22,14 @@ public:
 			port_id				GetPort(const char* signature,
 									const char* name);
 
+			status_t			StartSession(const char* login,
+									const char* password);
+
+	class Private;
+
 private:
+	friend class Private;
+
 			void				_InitMessenger();
 
 private:

headers/private/app/LaunchRosterPrivate.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2015 Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef _LAUNCH_ROSTER_PRIVATE_H
+#define _LAUNCH_ROSTER_PRIVATE_H
+
+
+#include <LaunchRoster.h>
+
+
+class BLaunchRoster::Private {
+public:
+								Private(BLaunchRoster* roster);
+								Private(BLaunchRoster& roster);
+
+			status_t			RegisterSession(const BMessenger& target);
+
+private:
+			BLaunchRoster*		fRoster;
+};
+
+
+#endif	// _LAUNCH_ROSTER_PRIVATE_H