WriteAt use pwrite syscall on *nix or WriteFile on Windows.

On Linux/Windows, these system calls always write to end of file in
append mode, regardless of offset parameter.

It is hard (maybe impossible) to make WriteAt work portably.

Making WriteAt returns an error if file is opened in append mode, we
guarantee to get consistent behavior between platforms, also prevent
user from accidently corrupting their data.

Fixes #30716

Change-Id: If83d935a22a29eed2ff8fe53d13d0b4798aa2b81
Reviewed-on: https://go-review.googlesource.com/c/go/+/166578Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>