init: move SELinux into enforcing mode.

When init starts up, immediately put SELinux into enforcing mode.

This is currently a no-op. We currently have everything in the
unconfined domain, so this should not break anything.
(if it does, I'll roll it back immediately)

If the kernel doesn't have SELinux support compiled in, then
don't try loading a policy and continue without SELinux protections.

Change-Id: Id0279cf82c545ea0f7090137b7566a5bc3ddd641