net/http: short writes with FileServer on macos [1.23 backport]

[gopherbot]

@ianlancetaylor requested issue #70000 to be considered for backport to the next 1.23 minor release.

@gopherbot Please open backport issues.

This causes inexplicable and difficult to avoid failures on BSD systems. We should backport the fix.

[gopherbot]

Change https://go.dev/cl/622696 mentions this issue: [release-branch.go1.23] internal/poll: keep copying after successful Sendfile return on BSD

[gopherbot]

Change https://go.dev/cl/622697 mentions this issue: [release-branch.go1.23] internal/poll: handle the special case of sendfile(2) sending the full chunk

[neild]

Cherrypick should include both https://go.dev/cl/622696 (fix) and https://go.dev/cl/622697 (followup tweak to the fix).

[gopherbot]

Closed by merging CL 622696 (commit 6ba3a8a) to release-branch.go1.23.