os: use unsafe.{Slice,StringData} instead of unsafeheader package

os: use unsafe.{Slice,StringData} instead of unsafeheader package

Change-Id: I213b078effa4b7049c44498d651de5b938e5404b
Reviewed-on: https://go-review.googlesource.com/c/go/+/428779
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Run-TryBot: hopehook <hopehook@golangcn.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>