magic: Add MagicPacket.send_to method

On the off chance that the magic packet needs to be sent to a
destination other than port 9 on the subnet-directed broadcast address,
the `send_to` method is now available on the `MagicPacket` structure.
The `send` method is retained, and now simply delegates to the `send_to`
method with a hard-coded socket address.

src/magic.rs

@@ -13,9 +13,13 @@ impl MagicPacket {
     }
 
     pub fn send(&self) -> io::Result<usize> {
+        self.send_to("255.255.255.255:9")
+    }
+
+    pub fn send_to<A: net::ToSocketAddrs>(&self, addr: A) -> io::Result<usize> {
         let socket = net::UdpSocket::bind("0.0.0.0:0")?;
         socket.set_broadcast(true)?;
-        socket.connect("255.255.255.255:9")?;
+        socket.connect(addr)?;
         let buf: Vec<u8> = self.iter().collect();
         socket.send(&buf[..102])
     }