cp,mv: reinstate that -n exits with success if files skipped

cp,mv: reinstate that -n exits with success if files skipped

* src/cp.c (main): Adjust so that -n will exit success if skipped files.
* src/mv.c (main): Likewise.
* doc/coreutils.texi (cp invocation): Adjust the description of -n.
* src/system.h (emit_update_parameters_note): Adjust --update=none
comparison.
* tests/cp/cp-i.sh: Adjust -n exit status checks.
* tests/mv/mv-n.sh: Likewise.
* NEWS: Mention the change in behavior.
Fixes https://bugs.gnu.org/62572