net,os: consolidate poll.SendFile sending until EOF with 0

net,os: consolidate poll.SendFile sending until EOF with 0

We've already use size==0 to indicate sending until EOF for
poll.SendFile on non-Linux platforms: Windows/*BSD/macOS/Solaris.

Let's harmonize Linux with others, making poll.SendFile on Linux
match its comment.

Change-Id: Ibfe9c9aa8f16bc37812afce9f95995c715cce0bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/623057
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>