kern: add an option for preserving the early kenv

[kevans]

Summary

Some downstream configurations do not store secrets in the
early (loader/static) environments and desire a way to preserve these
for diagnostic reasons. Provide an option to do so.

[jhb]

sys/conf/options

1025

Given there's only a single option, I don't know that we need this single-line header. I don't really anticipate a large number of kenv options that warrant a header.

1029

I would drop "theoretically" as it's too dismissive. One could perhaps say, "potentially" but that's only slightly better. If we wanted to provide a caveat I think the caveat should explain which situations aren't insecure. For example, something like "This option is insecure except in controlled environments where the static environment's contents are known to be safe."

[kevans]

Address commentary from jhb