Separate the MD5 code into its own module.

Separate the MD5 code into its own module.

Add -k/-K to have cpdup use the experimental FSMID to detect changes,
allowing it to shortcut whole directory subhiearchies.

It should be noted that using cpdup with a NFS or other remote filesystem
as the source with this option will not work, since the NFS client cannot
currently detect if the server (or some other client) has modified a file
or directory.