Mk/Uses/uniquefiles.mk: Clearer and less ambiguous output

Make output messaging clearer about what uniquefiles is doing
especially with regard 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"

Differential Revision: https://reviews.freebsd.org/D2706
Reviewed by: mat
Approved by: mat (portmgr)