Use of libsmack for operations

This commit focuses on using libsmack to perform its operations.
The operations done by libsmack are: validating, reading and writing
labels.

It also adds the isolation of the usage message and the use of
the predefined key values from <linux/xattr.h>

Change-Id: Id67698d823c71ebdc4aeced4c371dcb95f7c7ea9
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>