set_drive_state: update subdisk states more correctly.
set_drive_state: update subdisk states more correctly.

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

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase