kldunload kernel: How should the kernel behave when it is requested to unload itself

Hi,

This is *NOT* joking.

While working on https://reviews.freebsd.org/D42527 I realized the
module kernel also has userrefs, that is to say, userland can request
to unload kernel, aka `kldunload kernel`.

This is interesting. Well no doubt that the loader can unload kernel.
Then after the kernel is loaded and has been initialized (SYSINIT), how
should it behave when it get an unload request?

I'm proposing https://reviews.freebsd.org/D42530 to do not allow unloading
the kernel. It is by intuition.

What do you think ?


Best regards,
Zhenlei