Adding functions to set labels

This new functions allow to set the smack labels
either for a single key or for a set of keys.

Also, reformat source file using GNU coding style
of indent.

Change-Id: I29325723d0e7f3a68e39644a81760507e03d1113
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>