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/BindException.java

@@ -0,0 +1,21 @@
+/* Copyright (c) 2008-2014, Avian Contributors
+
+   Permission to use, copy, modify, and/or distribute this software
+   for any purpose with or without fee is hereby granted, provided
+   that the above copyright notice and this permission notice appear
+   in all copies.
+
+   There is NO WARRANTY for this software.  See license.txt for
+   details. */
+
+package java.net;
+
+public class BindException extends SocketException {
+  public BindException(String message) {
+    super(message);
+  }
+
+  public BindException() {
+    this(null);
+  }
+}