Allow a zone to be included with the ip address that is parsed when
using DefaultResolver's LookupHost or LookupIPAddr

Fixes #20790
Fixes #20767

Change-Id: I4e0baf9ade6a095af10a1b85ca6216788ba680ae
Reviewed-on: https://go-review.googlesource.com/79935
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>