The kernel domain exists solely on boot, and is used by kernel threads.
Because of the way the system starts, there is never an entrypoint for
that domain, not even a file on rootfs. So tighten up the neverallow
restriction.

Remove an obsolete comment. The *.rc files no longer have a setcon
statement, and the transition from the kernel domain to init occurs
because init re-execs itself. The statement no longer applies.

Test: bullhead policy compiles.
Change-Id: Ibe75f3d25804453507dbb05c7a07bba1d37a1c7b