fixing the output for unassigned/disabled mouse buttons:

The configuration unassigned/disabled buttons were originally displayed with an empty string when using e.g. "ratslap -p f3", while their internal descriptor is NONE, which has to be used to disable/remove assignment using e.g. "ratslap -m f3 -8 NONE". This causes incompatibilities with the GUI tool "rattrap", which is incapable of showing disabled buttons or to disable buttons because they are not reported as "NONE" as required by "ratslap". This fix changes the behaviour of ratslap, disabled/unassigned buttons will be displayed as NONE instead of empty string, allowing to fix rattrap and enabling rattrap to remove assignments of buttons also.