syscall: support more flags when opening directories on Windows
syscall.Open was artificially limiting the flags that were eligible
to open directories on Windows. This change extend the cases where we
pass FILE_FLAG_BACKUP_SEMANTICS to all flag combinations allowed by
Unix.
Change-Id: Ia7c083bcba070f92ea61c6d67487bdefd0d99546
Reviewed-on: https://go-review.googlesource.com/c/go/+/619295
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Damien Neil <dneil@google.com>