ctdb-recovery: Use single char ASCII numbers for status from child

ctdb-recovery: Use single char ASCII numbers for status from child

  '0' = Child took the mutex
  '1' = Unable to take mutex - contention
  '2' = Unable to take mutex - timeout
  '3' = Unable to take mutex - error

This is a straightforward API.  When the child is generalised to an
external helper then this makes it easier for a helper to be, for
example, a simple script.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>