master - thin: Support thin_check --clear-needs-check-flag.

[Alasdair G Kergon <agk@redhat.com>]

On Mon, Apr 07, 2014 at 02:04:48PM +0100, Alasdair G Kergon wrote:
>   - Under precisely what circumstances would a user find a 'needs check' flag needs clearing by lvm?
 
After a bit more discussion, I think documentation is the best answer
for now.

Proper recovery from the current failure modes (disk i/o error, out of
metadata space etc.) that lead to this flag getting set are nearly
always still going to require manual intervention anyway, and so I don't
think clearing the flag automatically gains us anything from the lvm2
point of view until we can automatically distinguish between the failure
modes and handle them.

Alasdair