MFC 234190,234196,234280:

MFC 234190,234196,234280:
- Extend the KDB interface to add a per-debugger callback to print a
  backtrace for an arbitrary thread (rather than the calling thread).
  A kdb_backtrace_thread() wrapper function uses the configured debugger
  if possible, otherwise it falls back to using stack(9) if that is
  available.
- Replace a direct call to db_trace_thread() in propagate_priority()
  with a call to kdb_backtrace_thread() instead.

git-svn-id: svn://svn.freebsd.org/base/stable/9@235741 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f