net: force LookupAddr results to be rooted DNS paths even in the case of local source

net: force LookupAddr results to be rooted DNS paths even in the case of local source

The builtin name resolver using various resolution techniques is a bit
complicated and we sometimes fotget to take care of all the go and cgo
code paths and exchanging information to local and remote sources. This
change makes LookupAddr return absolute domain names even in the case of
local source.

Updates #12189.
Fixes #12240.

Change-Id: Icdd3375bcddc7f5d4d3b24f134d93815073736fc
Reviewed-on: https://go-review.googlesource.com/17216
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>