Prompt once again for `mv -i A B' when A and B are hard links

Prompt once again for `mv -i A B' when A and B are hard links
to the same file.  This fixes a bug introduced by my 2003-04-04
(coreutils-5.0.1) change.

(abandon_move): New function, factored out of
copy_internal, now that this code is being used from two places.
(copy_internal): Perform the same interactive-related test for
whether it's alright to proceed and (usually) overwrite the
destination file.