net/http: refactor ResponseWriter.ReadFrom to permit splice on Linux

Rather than probe and guess if sendfile will work inside ResponseWriter.ReadFrom(src),
this change fixes the underlying issue of starting to respond before src is readable
We'll no longer send a status OK if a header has not yet been written and reading
from src is destined to fail. This small change implicitly takes care of the need for
the server to sniff the response body to determine the Content-Type.

This allows splice to work on Linux when src is a socket or any non-regular file that's spliceable.

The extra read of 512 bytes may raise an objection, and that's fair, but
we're already swapping some syscall prep work for another and a read of
512 probably will not impact the overall performance. For shorter
bodies, there's likely less setup time. A little initial slop is not too
unusual in zero copy network code, and sometimes actually helps.

Fixes #40888

Change-Id: I4a8e2ad0ace1318bae66dae5671d06ea6d4838ed
GitHub-Last-Rev: 097364ea866613d103a31e2247b44f4a12077f9e
GitHub-Pull-Request: golang/go#40903
Reviewed-on: https://go-review.googlesource.com/c/go/+/249238


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>