[release-branch.go1.23] net: don't use sendfile for irregular files; handle EOPNOTSUPP/ENOTSUP
This is not a cherry pick, because the code has changed on tip.
For #70763
Fixes #70789
Change-Id: If9fcfee17e86a746cf8c72293dc34f80b430f6e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/635397
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>