mv,cp: preserve symlink xattrs when copying across file systems

mv,cp: preserve symlink xattrs when copying across file systems

* src/copy.c (copy_internal): Include the copy_attr() call for symlinks.
This should not dereference symlinks, since llistxattr() is used
in attr_copy_file() in libattr, and so should copy all but the filtered
extended attributes.  Note we don't just move the copy_attr() call
before the set_owner() call, as that would break capabilities
for non symlinks.
* tests/cp/cp-mv-enotsup-xattr.sh: Add a test case.
* NEWS: Mention the bug fix.
Fixes http://bugs.gnu.org/16131