The unix and windows getaddrinfo calls return a zone with IPv6
addresses. IPv6 link-local addresses returned are only valid on the
given zone. When the zone is dropped, connections to the address
will fail. This patch replaces IP with IPAddr in several internal
resolver functions, and plumbs through the zone.

Change-Id: Ifea891654f586f15b76988464f82e04a42ccff6d
Reviewed-on: https://go-review.googlesource.com/5851Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>