cp: treat selinux warnings consistently
* src/copy.c (copy_reg): Suppress SELinux ENOTSUP warnings consistently
between the destination being present or not. Previously we did
not suppress ENOTSUP messages when the destination was present.
(copy_internal): Use the same ENOTSUP supression method as
copy_reg() even though the issue was not seen in this case.
* tests/cp/cp-a-selinux: Add a test case for the issue and
group the other test cases in the file more coherently.
* tests/cp/cp-mv-enotsup-xattr: Do the same check for xattr
warnings, even though they did not have the issue.