Stop writing big-endian checksums into the little-endian GPT

Stop writing big-endian checksums into the little-endian GPT
header fields gh_csum and gh_part_csum.

Constrain kernel to accepting only correct little-endian
checksums.

Temporarily allow fdisk(8) to read either endian GPTs so that
big-endian GPTs can be made correct by a simple 'fdisk -e' &&
'w'.

Fixes inter-architecture, inter-OS GPT portability and GPT
fdisk(8) on big-endian architectures. Broken since initial GPT
implementation.

Suggestions and ok kettenis@