Fix omxplayer on multi user envs.

An user reported me that if you launch omxplayer with user A and when it finish you launch again omxplayer but with user B you can't control it (play/pause/stop/...) because the dbus daemon and tmp files are owned by A.

This commit should fix but I can't test right now (probably can't test it for some weeks, so if you can check it would be nice).

Here an extract of the email I've received from the user:
```
I've run into a bug that has an easy workaround once I figured out
what was going on, but was vexing for a little while: if a user runs
omxplayer, the wrapper will create the files
/tmp/omxplayerdbus{,.pid}, owned by that user. Even after that
omxplayer is no longer running, as long as those files exist another
user cannot use omxplayer -- the player will start, but the controls
(play/pause/stop etc) won't work.
```