Bug#556610: incremental checkarray every night instead of a full monthly one

[bug556610 at arcor.de]

Some clarification for point 3) from above:

/var/lib/mdadm/autocheck-running has two purposes.

* The file is used to ensure autocheck interruption is only done once (either by
  the sleeping checkarray, by starting a manual check, or by the
  shutdown script), to avoid that the later --interrupt deletes all states
  (since it sees no checks running anymore).

* The file should contain (list) the arrays that are being checked automatically.
  This should avoid having to determine running checks,
  and enable checkarray to simply use that list instead
  to interrupt any autochecks before continuing to trigger a manual check or
  the next day's auto checks. (While leaving all manual checks running that may
  have been started already.)

The last sentence of 3) should thus read:
...proceed with the --interrupt action for [all auto checks mentioned in the file,
if no autocheck-running file present anymore simply exit.



Correcting and making the second sentence of 4) explicit:

If an autocheck-running file is present, checkarray --interrupt should remove it,
and save the state of, and stop all running checks.


5) (to make this explicit) 
If checkarray is called while an autocheck-running file is already present,
it should interrupt all still running checks listed in the autocheck-running file
before continuing with triggering manual checks or resuming (a possibly new set of)
automatic checks.