extent-scan: adjust naming and formatting
* src/extent-scan.h [struct extent_scan]: Rename member:
s/hit_last_extent/hit_final_extent/. "final" is clearer,
since "last" can be interpreted as "preceding".
Rename extent-scan functions to start with extent_scan_.
* src/Makefile.am (copy_sources): Also distribute extent-scan.h.
* src/extent-scan.c: Don't include error.h or quote.h. Neither is used.
* src/copy.c: shorten a comment to fit in 80 columns
* src/extent-scan.c, src/extent-scan.h: Correct formatting.