Kernel free() and NULL-safety

[Michael McConville <mmcconv1%sccs.swarthmore.edu@localhost>]

Is it decided that free/kern_free shouldn't be NULL-safe? That seems odd
and potentially risky to me, especially because POSIX specifies
userland's free() as NULL-safe. This change would also simplify the
kernel code by removing innumerable NULL checks.

I know that Linux and OpenBSD have NULL-safe kernel free(), so it may
also pose a risk when adapting or importing code.