cp,mv: skipping due to -u is success, not failure

cp,mv: skipping due to -u is success, not failure

This reverts the previous change, so that when a file
is skipped due to -u, this is not considered a failure.
* doc/coreutils.texi: Document this.
* src/copy.c (copy_internal): If --update says to skip,
treat this as success instead of failure.
* tests/mv/update.sh, tests/cp/slink-2-slink.sh:
Revert previous change, to match reverted behavior.