[release-branch.go1.23] internal/poll: keep copying after successful Sendfile return...

[release-branch.go1.23] internal/poll: keep copying after successful Sendfile return on BSD

The BSD implementation of poll.SendFile incorrectly halted
copying after succesfully writing one full chunk of data.
Adjust the copy loop to match the Linux and Solaris
implementations.

In testing, empirically macOS appears to sometimes return
EAGAIN from sendfile after successfully copying a full
chunk. Add a check to all implementations to return nil
after successfully copying all data if the last sendfile
call returns EAGAIN.

For #70000
For #70020

Change-Id: I57ba649491fc078c7330310b23e1cfd85135c8ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/622235
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
(cherry picked from commit bd388c0216bcb33d7325b0ad9722a3be8155a289)
Reviewed-on: https://go-review.googlesource.com/c/go/+/622696