internal/poll: set the correct file offset in FD.Seek for Windows overlapped handles
Windows doesn't keep the file pointer for overlapped file handles.
To work around this, we keep track of the current offset ourselves
and use it on every Read/Write operation.
When the user calls File.Seek with whence == io.SeekCurrent, it expects
that the offset we keep track of is also accounted for, else the
the seek'ed value won't match the file pointer seen by the user.
Updates #74951.
Fixes #75081.
Change-Id: Ieca7c3779e5349292883ffc293a8474088a4dec7
Reviewed-on: https://go-review.googlesource.com/c/go/+/697275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>