vchoi_fork

[svn-r13872] WinXP x64 does not define the error code EWOULDBLOCK, but instead uses its own WSAEWOULDBLOCK.  Although the stream vfd isn't supported on Windows, this was requested by a user via bug #890.  This won't affect any other platforms, but should help, should we ever decide to support stream vfd on Windows.

Tested:
kagiso, linew, smirom (via h5committest)
NOT tested on Windows, as it is unsupported.