chsmack: Major improvements

- Option -L (--dereference) added to dereference symbolic links
- Option -d (--remove) added to remove labels
- Validation of the labels
- Check that -t (--transmute) is applied on directories
- Use of names defined in <linux/xattr.h>
- Documentation updated

Change-Id: I83bd820b8f486df221a3f9e02dc7fd96b64c3f3f
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>