Further cleanup on cpv script;

[fghalasz]

1. add back a ln_or_cp function and use it to cp only when ln fails. Deploy t5his function for all instances of linking/copying in the script;
2. better handling of case where the unversioned destination file does not exist but versioned destination file DOES exist. In this case creates a new next higher versioned file. Previously, in this case just created a new unversioned file and did not create a new higher version file.
3. more informative error messages

[masinter]

note that #1352 inconsistent

[fghalasz]

OK. Removed local declarations from cpv script to make it POSIX-compliant. Ready for re-review.