The first version of my reworked xattr patch. We now use a checksum

The first version of my reworked xattr patch.  We now use a checksum
on longer xattr values to look for changes, we request any needed
abbreviated values from the sender when they're needed, and we now
properly check if a file has xattr changes (so that we itemize it
correctly, and avoid hard-linking files that aren't 100% identical).