[release-branch.go1.20] net, os: net.Conn.File.Fd should return a blocking descriptor

Historically net.Conn.File.Fd has returned a descriptor in blocking mode.
That was broken by CL 495079, which changed the behavior for os.OpenFile
and os.NewFile without intending to affect net.Conn.File.Fd.
Use a hidden os entry point to preserve the historical behavior,
to ensure backward compatibility.

For #58408
For #60211
For #60217

Change-Id: I8d14b9296070ddd52bb8940cb88c6a8b2dc28c27
Reviewed-on: https://go-review.googlesource.com/c/go/+/496080
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
(cherry picked from commit b950cc8f11dc31cc9f6cfbed883818a7aa3abe94)
Reviewed-on: https://go-review.googlesource.com/c/go/+/496715
3 files changed
tree: 558bec9fc480681d1542ef1dbe0290934c0cff65
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. codereview.cfg
  11. CONTRIBUTING.md
  12. LICENSE
  13. PATENTS
  14. README.md
  15. SECURITY.md
  16. VERSION
README.md

The Go Programming Language

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