net: fix handling of Conns created by Resolver.Dial

The DNS client in net is documented to treat Conns returned by
Resolver.Dial which implement PacketConn as UDP and those which don't as
TCP regardless of what was requested. golang.org/cl/37879 changed the
DNS client to assume that the Conn returned by Resolver.Dial was the
requested type which broke compatibility.

Fixes #26573
Updates #16218

Change-Id: Idf4f073a4cc3b1db36a3804898df206907f9c43c
Reviewed-on: https://go-review.googlesource.com/125735


Run-TryBot: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>