syscall,os: move flags validation from os.OpenFile to syscall.Open
syscall.Open is the functions that maps Unix/Go flags into Windows
concepts. Part of the flag validation logic was still implemented
in os.OpenFile, move it to syscall.Open for consistency.
A nice side effect is that we don't have to translate the file name
twice in case of an access denied error.
Change-Id: I32c647a9a2a066277c78f53bacb45fb3036f6353
Reviewed-on: https://go-review.googlesource.com/c/go/+/619275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>