Updates to TskAutoDb::fsWalkUnallocBlocksCb to support grouping of
contiguous blocks of unallocated space into larger chunks whose size is
configurable.  These changes satisfy the first two bullets of the DoD for
AUT-924.
Set the unallocated 'chunk' size to be a hard-coded value of 500MB.
Updated TskAutoDb::fsWalkUnallocBlocksCb() to handle the case where chunk
size is set equal to zero, in which case all unallocated data for a volume
is put into a single file.