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Various updates to the kernel debugging chapter.

- Document vmcore.last and describe it as the way to find the most recent
  dump rather than the highest numbered dump.
- Document crashinfo and that it automatically runs to generate a
  core.txt.N file if core dumps are enabled in rc.conf.
- Add a section on testing kernel dumps via the debug.kdb.panic sysctl.
  Remove a later note about debug.kdb.panic from the DDB section.
- Remove any mention of gdb -k (for pre 5-3 kernels) and just talk about
  kgdb.
- Remove paragraph that talks about trying to find the kernel.debug file.
  Instead, recommand 'kgdb -n <N>' which does this lookup automatically,
  and specifically recommend 'kgb -n last' to open the most recent
  crash dump.  Mention the fallback of specifying the kernel and
  vmcore directly if needed.
- Remove example dump from FreeBSD 2.  It is generally no longer relevant.
  It used gdb -k which uses a different stack trace format as well as
  including a 'frame' command that doesn't existing kgdb.  (kgdb instead
  lets you switch to different threads and processes).
- Remove mention of old boot blocks that don't load debug symbols.  I think
  this was last relevant in FreeBSD 2.x or 3.x.
- Rework the description of 'boot -d' to assume the boot menu and explicitly
  mention 'boot -d' at the loader prompt.
- Document how to get stack traces of other threads in DDB.
- Fix a few references to gdb to reference kgdb instead.
- Replace 'call cpu_reset' with 'reset' for DDB.

Differential Revision:  https://reviews.freebsd.org/D14711

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