add a diffsyspkgs  call that diffs package list from two servers

This builds on my previous pull request.

Add the "-c diffsyspkgs -p server1,server2" command that takes the package list from both and reports those installed on server1 that do *not* exist on server2. Use case is "is server2 fully patched" or "why does software work on server1 but not on server2".

May I suggest you test this before you apply :)

chris