net: fix parsing literal IPv6 address with zone identifier in builtin dns stub resolver

Fixes #8619.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/132560043

src/pkg/net/dnsconfig_unix.go

@@ -42,7 +42,9 @@ func dnsReadConfig(filename string) (*dnsConfig, error) {
 				// One more check: make sure server name is
 				// just an IP address.  Otherwise we need DNS
 				// to look it up.
-				if ParseIP(f[1]) != nil {
+				if parseIPv4(f[1]) != nil {
+					conf.servers = append(conf.servers, f[1])
+				} else if ip, _ := parseIPv6(f[1], true); ip != nil {
 					conf.servers = append(conf.servers, f[1])
 				}
 			}

src/pkg/net/dnsconfig_unix_test.go

@@ -18,7 +18,7 @@ var dnsReadConfigTests = []struct {
 	{
 		name: "testdata/resolv.conf",
 		conf: dnsConfig{
-			servers:  []string{"8.8.8.8", "2001:4860:4860::8888"},
+			servers:  []string{"8.8.8.8", "2001:4860:4860::8888", "fe80::1%lo0"},
 			search:   []string{"localdomain"},
 			ndots:    5,
 			timeout:  10,

src/pkg/net/testdata/resolv.conf

@@ -3,5 +3,6 @@
 domain localdomain
 nameserver 8.8.8.8
 nameserver 2001:4860:4860::8888
+nameserver fe80::1%lo0
 options ndots:5 timeout:10 attempts:3 rotate
 options attempts 3