selinux: make use of cleanup gcc magic

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
mac: rename all calls that apply a label mac_{selinux|smack}_apply_xyz(), and all that reset it to defaults mac_{selinux|smack}_fix()

Let's clean up the naming schemes a bit and use the same one for SMACK
and for SELINUX.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
smack: rework smack APIs a bit

a) always return negative errno error codes
b) always become a noop if smack is off
c) always take a NULL label as a request to remove it

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>