net: correct DNS configuration

Set defaults to the same values as various C DNS resolver libraries.
Use the timeout value from the configuration.
Fixes #632.

R=rsc
CC=golang-dev
https://golang.org/cl/223094

src/pkg/net/dnsclient.go

@@ -62,7 +62,7 @@ func _Exchange(cfg *_DNS_Config, c Conn, name string) (m *_DNS_Msg, err os.Error
 			return nil, err
 		}
 
-		c.SetReadTimeout(1e9) // nanoseconds
+		c.SetReadTimeout(int64(cfg.timeout) * 1e9) // nanoseconds
 
 		buf := make([]byte, 2000) // More than enough.
 		n, err = c.Read(buf)

src/pkg/net/dnsconfig.go

@@ -32,8 +32,8 @@ func _DNS_ReadConfig() (*_DNS_Config, os.Error) {
 	conf.servers = make([]string, 3)[0:0] // small, but the standard limit
 	conf.search = make([]string, 0)
 	conf.ndots = 1
-	conf.timeout = 1
-	conf.attempts = 1
+	conf.timeout = 5
+	conf.attempts = 2
 	conf.rotate = false
 	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
 		f := getFields(line)