Import existing /usr/bin/mdcheck (from mdadm bee package) into
repository as mdcheck.safe.

Sometimes, when the check is paused by setting sync_action to "idle",
the array deadlocks. It can be continued by setting array_state to
active.

If it takes more 10 seconds to set the sync_action to idle and the array
state is write-pending, then try to continue the array by setting its
state to active.

Use variant of mdcheck which is supposed to work arpound the "md_raid:
mdX_raid6 looping after sync_action "check" to "idle" transition" bug.

Reenable montly check staring from the 13. of every month.