net: use /etc/hosts first when looking up IP addresses using native Go's dns resolver

Previously /etc/hosts would be ignored altogether, this change returns matching results
from that file without talking to a DNS server.

R=rsc
CC=golang-dev
https://golang.org/cl/5061042

src/pkg/net/dnsclient_unix.go

@@ -215,6 +215,18 @@ func goLookupHost(name string) (addrs []string, err os.Error) {
 // depending on our lookup code, so that Go and C get the same
 // answers.
 func goLookupIP(name string) (addrs []IP, err os.Error) {
+	// Use entries from /etc/hosts if possible.
+	haddrs := lookupStaticHost(name)
+	if len(haddrs) > 0 {
+		for _, haddr := range haddrs {
+			if ip := ParseIP(haddr); ip != nil {
+				addrs = append(addrs, ip)
+			}
+		}
+		if len(addrs) > 0 {
+			return
+		}
+	}
 	onceLoadConfig.Do(loadConfig)
 	if dnserr != nil || cfg == nil {
 		err = dnserr