vdagent: Add daemonizing support, daemonize by default
gnome-session will wait for autostartup "apps" marked with
X-GNOME-Autostart-Phase=Initialization to signal they've completed starting
or do an exit 0, before continuing with the next startup phase. Since we were
doing neither, it would wait for a timeout until continueing causing a
noticable delay before gdm showing the greeter, and after login. By
daemonizing immediately after startup we avoid this delay without needing
to go talk XSMP or something similar.