os: remove stuttering return value names

os: remove stuttering return value names

Old style. Make it compliant with our code review comments document.

Also, make WriteString's return parameter named 'n', not 'ret', for
consistency.

Noticed during another documentation review.

Change-Id: Ie88910c5841f8353bc5c0152e2168b497578e15e
Reviewed-on: https://go-review.googlesource.com/12324
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>