bug#61105: cp/mv: want a fatal --no-clobber

[Paul Eggert]

Better, I think, would be to change cp -n to be compatible with FreeBSD, 
where 'cp -n A B' exits with status 1 if B exists. This matches what 
FreeBSD 'cp -i A B' does when you say "no". This would be more useful 
than what GNU cp does, and the FreeBSD cp -i behavior conforms to POSIX 
whereas GNU cp -i (I think accidentally) does not. This would be simpler 
than adding a fatal --no-clobber, and would encourage portability a bit.

The reason it's a POSIX conformance issue is that POSIX says exit status 
zero means "All files were copied successfully." which "cp -n A B" does 
not do when B exists.

Similarly for 'mv' (though FreeBSD mv -i does not conform to POSIX here, 
unfortunately).