net: return rooted DNS names on Plan 9

net: return rooted DNS names on Plan 9

This change returns rooted DNS names on Plan 9,
for consistency with other operating systems.

Updates #12193.

Change-Id: If983920c5b9a8f67d4ccb51bb295fac8dfb87e88
Reviewed-on: https://go-review.googlesource.com/15581
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>