net: try /etc/hosts before loading DNS config.

On Mac X 10.6 /etc/resolv.conf is changed dynamically,
and may not exist at all when all network connections
are turned off, thus any lookup, even for "localhost"
would fail with "error reading DNS config: open
/etc/resolv.conf: no such file or directory". This
change avoids the error by trying to lookup addresses
in /etc/hosts before loading DNS config.

R=golang-dev, rsc1, rsc
CC=golang-dev
https://golang.org/cl/4431054

src/pkg/net/dnsclient.go

@@ -313,16 +313,16 @@ func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err os.Erro
 // depending on our lookup code, so that Go and C get the same
 // answers.
 func goLookupHost(name string) (addrs []string, err os.Error) {
-	onceLoadConfig.Do(loadConfig)
-	if dnserr != nil || cfg == nil {
-		err = dnserr
-		return
-	}
 	// Use entries from /etc/hosts if they match.
 	addrs = lookupStaticHost(name)
 	if len(addrs) > 0 {
 		return
 	}
+	onceLoadConfig.Do(loadConfig)
+	if dnserr != nil || cfg == nil {
+		err = dnserr
+		return
+	}
 	ips, err := goLookupIP(name)
 	if err != nil {
 		return