windows: fix fs_write with nbufs > 1 and offset

When multiple bufs are specified, overlapped needs to be advanced manually between each write.  Without this, each buf will be written to the same position (if offset is specified.)

[`ReadFile()`] (http://msdn.microsoft.com/en-us/library/windows/desktop/aa365467%28v=vs.85%29.aspx) automatically advances, but [`WriteFile()`] (http://msdn.microsoft.com/en-us/library/windows/desktop/aa365747%28v=vs.85%29.aspx) does not.

Since the buf flow seems to have been introduced only in unstable releases, I suppose no one should've been accidentally depending on the previous behavior.

-[Unknown]