Add a kgdb python script to extract bbl from kernel dumps

A kgdb script allows us to be relatively resilient to kernel structure
changes, with a lower burden for updates when they happen than a C tool.

tuexen@ requested we ship this script in a source distribution. Making
it easier for users to extract useful debugging information from a core.
It requires kgdb and python3 and would ideally be a lldb lua script, but
there is more work needed on lldb lua before we can do that.

Add the script as is. A single script we could run against a core would
be nice, but I don't want to let that block making this tool more
available.

Reviewed by: teuxen, rrs
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48705