net: send EDNS(0) packet length in DNS query

Advertise to DNS resolvers that we are willing and able to accept up
to 1232 bytes in a DNS packet. The value 1232 was chosen based on
https://dnsflagday.net/2020/.

For #6464
For #21160
For #44135
For #51127
Fixes #51153

Change-Id: If9182d5210bfe047cf0a4d46163effc6812ab677
Reviewed-on: https://go-review.googlesource.com/c/go/+/386016


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>