o Minor bugfixes (logging):
    - Disable backtrace signal handlers when shutting down tor.
      Fixes bug 31614; bugfix on 0.2.5.2-alpha.
    - Add a missing check for HAVE_PTHREAD_H, because the backtrace code uses
      mutexes. Fixes bug 31614; bugfix on 0.2.5.2-alpha.
  o Documentation:
    - Explain why we can't destroy the backtrace buffer mutex. Explain why
      we don't need to destroy the log mutex.
      Closes ticket 31736.