basic_file_body is specialized for file_win32:
When serializing file bodies to stream sockets on Windows,
the native platform function ::TransmitFile is used, improving
performance.
These types now perform error-based initialization in
a separate init() functions instead of in the constructor.
Actions Required:
* Modify instances of user-defined BodyReader and BodyWriter
types to perfrom two-phase initialization, as per the
updated documented type requirements.
Vinnie Falco