copy: make backup files more reliably

copy: make backup files more reliably

* NEWS, doc/coreutils.texi (Backup options): Document the change.
* bootstrap.conf (gnulib_modules): Add backup-rename.
* src/copy.c (copy_internal): Silently switch to numbered backups
if a simple backup might lose data.  Use backup_file_rename
to avoid races with numbered backups.
* tests/cp/backup-is-src.sh, tests/mv/backup-is-src.sh:
Adjust to match new behavior.