client/cmd/rpc: isLocalHost: check [::1] IPv6 host
client/cmd/rpc: isLocalHost: check [::1] IPv6 host

R=dnj@chromium.org, vadimsh@chromium.org

Review URL: https://codereview.chromium.org/1653563002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go