various refinements to network implementation

The main idea is to make DatagramChannel and *SocketChannel behave in
a way that more closely matches the standard, e.g. allow binding
sockets to addresses without necessarily listening on those addresses
and accept null addresses where appropriate.  It also avoids multiple
redundant DNS lookups.

This commit also implements CharBuffer and BindException, and adds the
Readable interface.

classpath/java/net/Socket.java

@@ -191,6 +191,9 @@ public class Socket implements Closeable, AutoCloseable {
 		outputStream.close();
 	}
 	
+        public SocketAddress getRemoteSocketAddress() {
+                 throw new UnsupportedOperationException();
+        }
 	
 	@Override
 	protected void finalize() throws Throwable {