If an illegal header write is detected, find the first caller outside of
net/http using runtime.CallersFrames and include the call site in the log
message.

Fixes #18761

Change-Id: I92be00ac206c6ebdd60344ad7bf40a7c4c188547
Reviewed-on: https://go-review.googlesource.com/c/130997Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>