mdcheck: new script to help with regular checks of md arrays.

mdcheck: new script to help with regular checks of md arrays.

This script allows arrays to be 'checked' for a limited amount
of time on a regular basis.

For example, running

 mdcheck --duration 6hours

early every Sunday morning and

 mdcheck --continue 6hours

ever other morning will check all arrays every week, but if that take
more than 6 hours, will won't run into the day, but will be continued
the next morning, and the next ... etc.

Signed-off-by: NeilBrown <neilb@suse.de>