copy: go back to failing 'cp --backup a~ a'

copy: go back to failing 'cp --backup a~ a'

Suggested by Kamil Dudka in:
http://lists.gnu.org/archive/html/coreutils/2017-07/msg00072.html
* NEWS: Document the changed nature of the fix.
* doc/coreutils.texi, tests/cp/backup-is-src.sh:
* tests/mv/backup-is-src.sh: Revert previous change.
* src/copy.c (source_is_dst_backup): New function.
(copy_internal): Use it.  Fail instead of falling back on numbered
backups when it looks like the backup will overwrite the source.
Although this reintroduces a race, it's more compatible with
previous behavior.