Improved getting and setting

Adds the function 'smack_set_label_to_path' to the library.
That new function validate the label before setting it.

Fixes a bug in function 'smack_new_label_from_path' whose
specification stated that the length of the label is returned
but that returned zero. Also take into account a strange
behaviour of the LSM Smack to returns the effective length.

Change-Id: Ia5e3a7f59691d124984721e5392e70322426515b
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>