For information on FlushFileBuffers(), see the msdn document
at msdn.microsoft.com/en-us/library/aa364439(VS.85).aspx

Note that Windows 2000 is shown as the minimum windows version
to support FlushFileBuffers(), so if we wish to support Win9X
and NT4, we will need to add code to dynamically check if
kernel32.dll contains the function.

The only error return mentioned in the msdn document is
ERROR_INVALID_HANDLE, which is returned if the file/device
(eg console) is not buffered. The fsync(2) manpage says that
EINVAL is returned in errno, if "fd is bound to a special
file which does not support synchronization".

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>