Fix a missing checksum validation in pic12 code

Unless I'm missing some edgecase reason for pic12executive not checking parameter checksums (or retrying commands if the receiving end sends a parameter checksum failure)

Also:
* Move status code definitions to a common location
* Create some handy MIB slave return macros.

We should really de-duplicate the MIB code as much as possible, there's still a lot of duplicated logic that could easily get out-of-sync (as it did here)