vol: allow non-dafs volume utils to attach with V_READONLY again

Allow non-fileserver, non-dafs, programs to attach volumes with the
V_READONLY mode again. This was lost during the code changes for
dafs.

The caller sends a fssync request to the fileserver, which updates the
on-disk contents of the volume headers, before the caller reads the
volume headers, allowing the caller to have the most recent info about
the volume. The fileserver still has the volume in use.

Later in the attachment process, the inUse check is skipped for the case
of a non-fileserver process which is attaching the volume using the
V_READONLY mode, otherwise the attachment would incorrectly mark the
volume as needing to be salvaged.

Note: The mode checks in VMustCheckOutVolume() are correct. We must
checkout the volume when attaching with the V_READONLY mode. This
fix updates the VShouldCheckInUse(), in which an additional
exception was added to cover the case for V_READONLY mode from a non-
fileserver process.

Note: A check is added in the dafs version of attach to avoid overwriting the
inUse field when a volume utility is attaching a volume in V_READONLY mode.
Currently, V_READONLY is not used by dafs, but this was done to avoid future
errors.

Change-Id: I09d3af1e74087d0627399392e662e5075d41cdeb
Reviewed-on: http://gerrit.openafs.org/8339
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
fix stale volume info from vos examine (non-dafs)

A volume examine on a non-dafs volume server/fileserver can show old
information, including old volume update time, for up to about 20
minutes. The non-dafs volume server reads the volume information
from the volume headers, which are updated by the fileserver only
periodically to avoid excessive i/o.

Before dafs, when the volume server performed a volume examine, the
volume server would send a fssync command to the fileserver with the
request FSYNC_NEEDVOLUME and mode V_READONLY. The fileserver writes
the current memory contents to disk on this fssync command. The
volume server would then attach the volume, reading the current
volume data.

The dafs volume/fileserver avoids this extra i/o by using a new set
of fssync commands to retrieve the volume information from the
fileserver. However, the non-dafs volume server does not use the new
fssync commands and reads the volume headers from disk.

Revert the volume attachment processing for the non-dafs volume
server to request the volume with the V_READONLY mode. This causes
the fileserver to update the volume headers, allowing the volume
server to read the up to date volume header data.

Sadly, this adds another dafs ifdef to the already twisty maze of
passages that all look alike.

This changes the volserver to use the V_READONLY attachment mode
only for the case of getting a single volume, as that what was
done in 1.4.x.

Change-Id: Ibee25bfcfb087e73ccbc17e181a99da49a7751ae
Reviewed-on: http://gerrit.openafs.org/8327
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>