use MSG return code consistently, some comments
Switch MPSystem I.face to use StgWord8 (CpComm still broken)

This switches the messaging interface MPSystem.h to use unsigned
char (available as StgWord8) and byte sizes in MP_send and MP_recv.
(plus some other changes while going through the files)
This change is necessary to make the Windows CpComm way work on
64bit-Windows where sizeof(int) == sizeof(long) != sizeof(StgWord).

CpComm takes more work and will be fixed in a separate following commit.
             THE COPY WAY IS BROKEN WITH THIS COMMIT.