More porting work to label_ops_pre()/label_ops_post()
This also enables labelling of the /etc/localtime symlink in
systemd-firstboot, which should address one small facet of #37857
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().