Rewritten shake down of bucket file.

Rewritten shake down of bucket file.

  o  Replaced read and write buffers by a single shared buffer.
     This should be somewhat faster (with the same size of memory invested
     to buffers).
  o  If ShakeSecurity is set to 2, shaking down should be reliable under
     all circumstances, including server reboots and broken bucket files.
     Buckettool -F still needs to be run after a failed shakedown and
     oid's need to be synchronized with the outside world, but no buckets
     will be lost (only some of them may be duplicated).
  o  The callback function (`the kibitz') is now allowed not only to decide
     which buckets will be kept, but also to alter contents of the buckets
     provided that it won't enlarge the bucket.

I tried to be very careful and tested the new routine thoroughly, but since
it's a pretty critical place, I would be very happy if somebody checks it
independently.