set_sd_state: update the state of a subdisk in a multi-plex volume
more correctly.
update_plex_state: Bring the plex up correctly when the last subdisk
comes up.
checksdstate: Update comments.
vpstate: Don't return an "up" state on a degraded, unattached plex.
start_object: Return a sensible error message when trying to revive a
subdisk whose drive is down. Previously it returned EBUSY.
Approved-by: jkh
5823cef910