Mk/Uses/uniquefiles.mk: Make output clearer and less ambiguous

[koobs]

Summary

Make output messaging clearer about what uniquefiles is doing
especially with regards to moving and linking (default links).

• Add "action" prefix (Move, Link) to operations performed
• Use the form "Creating unique files" for the stage summary
• Be clear that we're processing files that *need* prefixes or suffixes, and not moving files that *have* prefixes or suffixes"

Test Plan

None