install: allow the combination of --compare and --preserve-timestamps

install: allow the combination of --compare and --preserve-timestamps

* NEWS: Mention the improvement.
* src/install.c (enum copy_status): New type to let the caller know if
the copy was performed or skipped.
(copy_file): Return the new type instead of bool. Reduce variable scope.
(install_file_in_file): Only strip the file if the copy was
performed. Update the timestamps if the copy was skipped.
(main): Don't error when --compare and --preserve-timestamps are
combined.
* tests/install/install-C.sh: Add some test cases.