tor/changes/timersub_bug

  o Major bugfixes:
    - Provide correct replacements for the timeradd() and timersub() functions
      for platforms that lack them (for example, windows). The timersub()
      function is used when expiring circuits, timeradd() is currently unused.
      Patch written by Vektor, who also reported the bug. Thanks! Bugfix
      on 0.2.2.24-alpha/0.2.3.1-alpha, fixes bug 4778.