Refactored the udp*.go files, to remove redundancy, and to make them compile for Go on App Engine. (#451)

App Engine even though is Linux, does not allow referencing the syscall package. Thus I reorganised
the udp*.go files a little to make this work. As best I can the functionality is unchanged, and all
this continues to compiles on darwin, linux, windows, plan9 and appengine.

Namely:
* Moved all Linux specific code from udp.go into udp_linux.go (e.g setUDPSocketOptions)
* Deleted udp_plan9.go which was almost a copy of udp.go.
* Added build tags to stop appengine using the linux build, and instead using the udp_other.go.

udp.go

@@ -1,10 +1,9 @@
-// +build !windows,!plan9
+// +build !windows
 
 package dns
 
 import (
 	"net"
-	"syscall"
 )
 
 // SessionUDP holds the remote address and the associated
@@ -17,29 +16,6 @@ type SessionUDP struct {
 // RemoteAddr returns the remote network address.
 func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
 
-// setUDPSocketOptions sets the UDP socket options.
-// This function is implemented on a per platform basis. See udp_*.go for more details
-func setUDPSocketOptions(conn *net.UDPConn) error {
-	sa, err := getUDPSocketName(conn)
-	if err != nil {
-		return err
-	}
-	switch sa.(type) {
-	case *syscall.SockaddrInet6:
-		v6only, err := getUDPSocketOptions6Only(conn)
-		if err != nil {
-			return err
-		}
-		setUDPSocketOptions6(conn)
-		if !v6only {
-			setUDPSocketOptions4(conn)
-		}
-	case *syscall.SockaddrInet4:
-		setUDPSocketOptions4(conn)
-	}
-	return nil
-}
-
 // ReadFromSessionUDP acts just like net.UDPConn.ReadFrom(), but returns a session object instead of a
 // net.UDPAddr.
 func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {

udp_linux.go

@@ -1,4 +1,4 @@
-// +build linux
+// +build linux,!appengine
 
 package dns
 
@@ -15,6 +15,29 @@ import (
 	"syscall"
 )
 
+// setUDPSocketOptions sets the UDP socket options.
+// This function is implemented on a per platform basis. See udp_*.go for more details
+func setUDPSocketOptions(conn *net.UDPConn) error {
+	sa, err := getUDPSocketName(conn)
+	if err != nil {
+		return err
+	}
+	switch sa.(type) {
+	case *syscall.SockaddrInet6:
+		v6only, err := getUDPSocketOptions6Only(conn)
+		if err != nil {
+			return err
+		}
+		setUDPSocketOptions6(conn)
+		if !v6only {
+			setUDPSocketOptions4(conn)
+		}
+	case *syscall.SockaddrInet4:
+		setUDPSocketOptions4(conn)
+	}
+	return nil
+}
+
 // setUDPSocketOptions4 prepares the v4 socket for sessions.
 func setUDPSocketOptions4(conn *net.UDPConn) error {
 	file, err := conn.File()

udp_other.go

@@ -1,17 +1,15 @@
-// +build !linux,!plan9
+// +build !linux appengine
 
 package dns
 
 import (
 	"net"
-	"syscall"
 )
 
 // These do nothing. See udp_linux.go for an example of how to implement this.
 
 // We tried to adhire to some kind of naming scheme.
-
+func setUDPSocketOptions(conn *net.UDPConn) error                  { return nil }
 func setUDPSocketOptions4(conn *net.UDPConn) error                 { return nil }
 func setUDPSocketOptions6(conn *net.UDPConn) error                 { return nil }
 func getUDPSocketOptions6Only(conn *net.UDPConn) (bool, error)     { return false, nil }
-func getUDPSocketName(conn *net.UDPConn) (syscall.Sockaddr, error) { return nil, nil }

udp_plan9.go

@@ -1,34 +0,0 @@
-package dns
-
-import (
-	"net"
-)
-
-func setUDPSocketOptions(conn *net.UDPConn) error { return nil }
-
-// SessionUDP holds the remote address and the associated
-// out-of-band data.
-type SessionUDP struct {
-	raddr   *net.UDPAddr
-	context []byte
-}
-
-// RemoteAddr returns the remote network address.
-func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
-
-// ReadFromSessionUDP acts just like net.UDPConn.ReadFrom(), but returns a session object instead of a
-// net.UDPAddr.
-func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
-	oob := make([]byte, 40)
-	n, oobn, _, raddr, err := conn.ReadMsgUDP(b, oob)
-	if err != nil {
-		return n, nil, err
-	}
-	return n, &SessionUDP{raddr, oob[:oobn]}, err
-}
-
-// WriteToSessionUDP acts just like net.UDPConn.WritetTo(), but uses a *SessionUDP instead of a net.Addr.
-func WriteToSessionUDP(conn *net.UDPConn, b []byte, session *SessionUDP) (int, error) {
-	n, _, err := conn.WriteMsgUDP(b, session.context, session.raddr)
-	return n, err
-}

udp_windows.go

@@ -8,6 +8,8 @@ type SessionUDP struct {
 	raddr *net.UDPAddr
 }
 
+func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
+
 // ReadFromSessionUDP acts just like net.UDPConn.ReadFrom(), but returns a session object instead of a
 // net.UDPAddr.
 func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
@@ -25,10 +27,3 @@ func WriteToSessionUDP(conn *net.UDPConn, b []byte, session *SessionUDP) (int, e
 	return n, err
 }
 
-func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
-
-// setUDPSocketOptions sets the UDP socket options.
-// This function is implemented on a per platform basis. See udp_*.go for more details
-func setUDPSocketOptions(conn *net.UDPConn) error {
-	return nil
-}