Some preparation steps for SCSI pass-through support:
Some preparation steps for SCSI pass-through support:

- stack command and DCDB frames along with scatter-gather list into
one command block;
- use structure field pointers and offsetof() instead of evil pointer
arithmetics;
- add DCDB definitions.

No functional changes.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars