This commit adds a KeepAlive field to ListenConfig and uses it
analogously to Dialer.KeepAlive to set TCP KeepAlives per default on
Accept()

Fixes #23378

Change-Id: I57eaf9508c979e7f0e2b8c5dd8e8901f6eb27fd6
GitHub-Last-Rev: e9e035d53ee8aa3d899d12db08b293f599daecb6
GitHub-Pull-Request: golang/go#31242
Reviewed-on: https://go-review.googlesource.com/c/go/+/170678
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>