DEV messaging stage 1/4: Rearrange struct cdevsw and add a message port

DEV messaging stage 1/4: Rearrange struct cdevsw and add a message port
and auto-queueing mask.  The mask will tell us which message functions
can be safely queued to another thread and which still need to run in the
context of the caller.   Primary configuration fields (name, cmaj, flags,
port, autoq mask) are now at the head of the structure.  Function vectors,
which may eventually go away, are at the end.  The port and autoq fields
are non-functional in this stage.

The old BDEV device major number support has also been removed from cdevsw,
and code has been added to translate the bootdev passed from the boot code
(the boot code has always passed the now defunct block device major numbers
and we obviously need to keep that compatibility intact).