dedup: dedup finds and clones duplicate files in APFS

Say you have a "placeholder" graphic for <movie poster> in a very large media library tree.

All the identical <no poster yet> graphics in all the seperate movie directories can point to the same set of image bits.

When a poster is found and image data written to the pre existing clone, a new file is created ( copy on write ) and the file-data-pointer redirected to the new poster and away from the heaviliy shared placeholder poster.

There's a caveat:

    While dedup is primarily intended to be used to save storage by using clones, but provides -l and -s flags to replace duplicates with hard links or symbolic links respectively. Care should be taken when using these options, however.