cp: make -a option preserve xattrs, but with reduced diagnostics

cp: make -a option preserve xattrs, but with reduced diagnostics

* copy.c (copy_attr_by_fd): Reduce xattr diagnostics for 'cp -a'.
(copy_attr_by_name): Likewise.
* cp.c (main): Preserve xattrs with -a option, when possible.
* doc/coreutils.texi: Document that xattrs are preserved with
cp -a, with no added diagnostics.
* NEWS: Mention the change.
* tests/misc/xattr: Add tests for 'cp --preserve=all' and 'cp -a'.