os: make File.Readdir et al concurrency-safe

os: make File.Readdir et al concurrency-safe

Before, all methods of File (including Close) were
safe for concurrent use (I checked), except the three
variants of ReadDir.

This change makes the ReadDir operations
atomic too, and documents explicitly that all methods
of File have this property, which was already implied
by the package documentation.

Fixes #66498

Change-Id: I05c88b4e60b44c702062e99ed8f4a32e7945927a
Reviewed-on: https://go-review.googlesource.com/c/go/+/578322
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>