Adapt return values

[fridex]

Return values should be disjoint from ones from calls to other parts of the kernel. This will give us ability to clarify what went wrong (e.g. there was an error in decryption, not data record (not 0x17), etc). Maybe they will need to be remapped (e.g. if crypto API returns same error as kernel_{recv,send}msg()).

[djwatson]

I think only one new one is necessary for not a data record