ui: Check that autostart file exists
ui: Check that autostart file exists

Preferences should only claim that sharing is enabled when the autostart file
exists in addition to the enable_upnp setting.

Fixes confusion in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=703281

https://bugzilla.gnome.org/show_bug.cgi?id=698572

jku/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,Tarnyko/rygel,GNOME/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel