crashinfo: Print stack traces for all on-CPU threads

Add a python script which implements the bulk of this functionality.
Over time, this would ideally evolve into a library of python routines
which can be used to inspect kernel data structures and automate some
debugging tasks, similar to jhb's out-of-tree scripts, but written in a
somewhat nicer language and with better integration into the kgdb
command prompt.

Note that kgdb currently won't auto-load scripts in this directory.
This should perhaps change in the future. It probably also makes more
sense to have a crashinfo.py which provides all the kgdb output that we
want to include in core.txt, rather than having crashinfo.sh pipe in
several commands.

Reviewed by: avg, imp
Discussed with: jhb
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33817