machinetalk-protobuf: added machine unit fields

[machinekoder]

The status messages where lacking fields for machine units. Instead there have been unit scales for linear, angular and per axis. This was replaced by machine unit enums. It is required that values are sent and received in metric units in order to make unit conversion easy.

Since the relevant message ids in the messages have not changed the changes are backward compatible with older machinetalk-protobuf version. It is very likely patch and the corresponding counterpart in mkwrapper fixes problems on machines with imperial units that where related to values being sent in machine units. Especially metric machines will no be affected by this patch at all.

[mhaberler]

hm, I guess this PR needs rebasing and a bit of fixing

[machinekoder]

Rebased on master.