o Minor bugfixes:
    - Find more places in the code that should have been testing for
      invalid sockets using the SOCKET_OK macro. Required for a fix
      for bug 4533. Bugfix on 0.2.2.28-beta.
    - Detect attempts to build Tor on (as yet hypothetical) versions
      of Windows where sizeof(intptr_t) != sizeof(SOCKET).  Partial
      fix for bug 4533. Bugfix on 0.2.2.28-beta.

  o Code simplification and refactoring:
    - Use a TOR_INVALID_SOCKET macro when initializing a socket to an
      invalid value, rather than just -1.