Completely rework command processing:
Completely rework command processing:

- use separate callbacks to finish scsi and raid management commands;
- for raid management commands use dedicated dma-safe buffer;
- remove unused run queue;

The main goal of these changes is to fix a bug showing up on the lpinto's
machine where READCONF command fails because it needs more than 1-page
buffer and the buffer was not contiguos and controller doesn't support
scatter-gather for raid management commands.

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