label: Introduce LabelOps to do pre/post labelling operations

label: Introduce LabelOps to do pre/post labelling operations

By default, label_ops is initialized with a NULL pointer which translates
to noop labelling operations. In mac_selinux_init() and the new mac_smack_init(),
we initialize label_ops with a MAC specific LabelOps pointer.

We also introduce mac_init() to initialize any configured MACs and replace all
usages of mac_selinux_init() with mac_init().