Improved param command, show now allows to filter and set allows to set int params

This change is not complete (NuttX seems to lack strtof() or I failed to find it, the standard headers are in place), but it is functional with integer parameters.

In particular this really important line works:

param set MAV_TYPE 2

We should add that to all start scripts to make sure everything is booting as the correct system type, since this influences the arming behavior.

param show got also fixed to now properly filter, so if just one parameter is wanted, one can just query this one.

I recommend therefore to merge, even if only halfway done, but keep the branch in the tree in order to add floating point support. Since we have user reports suggesting the MAV_TYPE might end up doing bad things if missing, it might be a good idea to have this added to all startup scripts soon.