Intel10G: Offload checksums

Second edition of checksum offloading.  This time verifies if the packet is asking to be checksummed before meddling with it.

It's also a little more robust, uses less `ffi.cast()` of pointers and removes `lib.bits{}`, which only works on 32-bit integers.  (it's easy to extend them to 64 bits (just replace `0` and `1` with `0ULL` and `1ULL`), but all the NIC registers are 32 bits, so it would recast values all the time)