[omxplayer] Add option to choose omxplayer/dvdplayer automatically

On Helix dvdplayer is pretty usable on the Pi, and is the only option
in some use cases:

omxplayer does not support external sound cards (USB or I2S)
omxplayer does not support dvd menus
omxplayer does not support software codecs

Expecting the user to choose the right player is not ideal,
so make the switch automatically for these three cases.