mirrored drive NAS operation improved

Hi,

Frank added a new altboot(8) functionality to boot off kernel
in multiple drive NAS configuration.  NetBSD/sandpoint is
now able to detect it and attempts to boot choosing a valid
rootfs drive.  This feature improves disk exchange operation
in mirrored drive configation.

- continue to operate after removal of mirrored drive.
- continue to operate after fresh drive insertion.

Note that it's not known that any hot-swap-ready sandpoint
NAS ever exist so far, and NetBSD disk subsystem is short
to make it possible.  Given that caveat, the new feature is
benefitial to run mirrored drive NAS without any hussle to
adjust boot string on every disk exchange operation.

Toru Nishimura / ALKYL Technology