Direct dumped kernel cores

Hi everyone,

At Juniper we've been using a so-called 'rescue' kernel for dumping
vmcores directly to the filesystem after a panic.  We're now
contributing this feature, implemented by Klara Systems, to FreeBSD, and
looking for feedback. I posted a review
at https://reviews.freebsd.org/D47358 for anyone interested.

Interesting bits to keep in mind:
* It requires a 2-stage build process, one to build the rescue kernel,
  the other to build the main kernel, which embeds the rescue kernel
  inside its image.  This might need some further work.
* Thus far it's been implemented for amd64 and arm64, once proven out,
  other architectures (powerpc64/le, riscv64) can follow suit.
* Kernel environment bits to pass down to the rescue kernel are
  prefixed `debug.rescue.`, for instance
  `debug.rescue.vfs.root.mountfrom`.

There are many more details in the review summary.

We'd love to get feedback from anyone interested.

Thanks,
Justin Hibbits