Implemented single-session D-Bus

* Changed D-Bus session handing so a single dbus-daemon is started the first time omxplayer starts, and continues running forever. The socket address and pid are stored in /tmp/omxplayerdbus and /var/run/omxplayerdbus.pid respectively
* The first omxplayer instance that runs will use the D-Bus name org.mpris.MediaPlayer2.omxplayer, or the name specified by the --dbus_name argument
* If that name is not available (e.g. being used by another omxplayer instance), ".instance{pid}" will be appended