go.net/ipv4: change I/O method signatures on PacketConn and RawConn

Preserves both Read and Write signatures for the future usage.

R=dave, rsc
CC=golang-dev
https://golang.org/cl/6970048
diff --git a/ipv4/mocktransponder_test.go b/ipv4/mocktransponder_test.go
index 99759ef..4b57f18 100644
--- a/ipv4/mocktransponder_test.go
+++ b/ipv4/mocktransponder_test.go
@@ -37,10 +37,10 @@
 			c.SetTTL(i + 1)
 		}
 		c.SetDeadline(time.Now().Add(100 * time.Millisecond))
-		if _, err := c.Write(wb, nil, dst); err != nil {
+		if _, err := c.WriteTo(wb, nil, dst); err != nil {
 			t.Fatalf("ipv4.PacketConn.Write failed: %v", err)
 		}
-		_, cm, _, err := c.Read(rb)
+		_, cm, _, err := c.ReadFrom(rb)
 		if err != nil {
 			t.Fatalf("ipv4.PacketConn.Read failed: %v", err)
 		}
@@ -72,10 +72,10 @@
 			wh.Dst = dst.(*net.IPAddr).IP
 		}
 		c.SetDeadline(time.Now().Add(100 * time.Millisecond))
-		if err := c.Write(wh, wb, nil); err != nil {
+		if err := c.WriteTo(wh, wb, nil); err != nil {
 			t.Fatalf("ipv4.RawConn.Write failed: %v", err)
 		}
-		rh, _, cm, err := c.Read(rb)
+		rh, _, cm, err := c.ReadFrom(rb)
 		if err != nil {
 			t.Fatalf("ipv4.RawConn.Read failed: %v", err)
 		}