Reported by ghubcoder in #2, debug builds were broken due to
incorrect MAX_NARG macro - it should be COMM_MAX_NARG.

Looks like removing printf drops about 10kB.

Now whatever arrives will get stored, and then used up by as many stages
as possible.

...but not it's obvious - we can't assume each packet nicely
encapsulates a single stage, it might all arrive in one go.
So the current "recv" implementation can't work. d'oh!

Don't allow data which is too long.

Now isolated for easy re-use. Should split it into a separate library.

Based on the Pi example