[SlimDevices: Beta] Override default.map

[kdf]

>
> Some tracing indicates that all the .map files in the IR directory get
> processed - but Default.map last - so I can't override - only add.

correct order, wrong result.  The code operates on first found, so if the
function exists in a custom map, then that is selected.  Only if there is
nothing else does it fall to Default map.  This works the same for modes. 
If a function for a specified mode exists then that is used.  If nothing
is found then it falls to "common".

You do, however, have to have the custom map chosen in the player settings
or the only map file checked is Default.

-kdf