cp: fix --no-preserve=mode to not exit 1

cp: fix --no-preserve=mode to not exit 1

cp --no-preserve=mode exited 1 unconditionally.  Furthermore,
the tests which would have detected this error - namely
link-preserve.sh and reserve-mode.sh - failed to test
cp's exit code.

* src/copy.c (copy_reg): In the case x->explicit_no_preserve_mode,
do only set return_val to false iff the previous set_acl ()
failed.
* tests/cp/link-preserve.sh: Check cp's exit code.
* tests/cp/link-symlink.sh: Likewise.
* tests/cp/preserve-mode.sh: Likewise.
* NEWS: Mention the fix.

Bug introduced in commit v8.19-145-g24ebca6.

Reported by Florian Pritz in http://bugs.gnu.org/13119.