cp,ln,mv: when skipping exit with nonzero status

cp,ln,mv: when skipping exit with nonzero status

* NEWS, doc/coreutils.texi: Document this.
* src/copy.c (copy_internal):
* src/ln.c (do_link): Return false when skipping action due to
--interactive or --no-clobber.
* tests/cp/cp-i.sh, tests/cp/preserve-link.sh:
* tests/cp/slink-2-slink.sh, tests/mv/i-1.pl, tests/mv/i-5.sh:
* tests/mv/mv-n.sh, tests/mv/update.sh:
Adjust expectations of exit status to match revised behavior.