Network stack refactor

This introduces CNetAddr and CService, respectively wrapping an
(IPv6) IP address and an IP+port combination. This functionality used
to be part of CAddress, which also contains network flags and
connection attempt information. These extra fields are however not
always necessary.

These classes, along with logic for creating connections and doing
name lookups, are moved to netbase.{h,cpp}, which does not depend on
headers.h.

Furthermore, CNetAddr is mostly IPv6-ready, though IPv6
functionality is not yet enabled for the application itself.

src/compat.h

@@ -0,0 +1,42 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2011 The Bitcoin developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file license.txt or http://www.opensource.org/licenses/mit-license.php.
+#ifndef _BITCOIN_COMPAT_H
+#define _BITCOIN_COMPAT_H 1
+
+typedef u_int SOCKET;
+#ifdef WIN32
+#define MSG_NOSIGNAL        0
+#define MSG_DONTWAIT        0
+typedef int socklen_t;
+#else
+#include "errno.h"
+#define WSAGetLastError()   errno
+#define WSAEINVAL           EINVAL
+#define WSAEALREADY         EALREADY
+#define WSAEWOULDBLOCK      EWOULDBLOCK
+#define WSAEMSGSIZE         EMSGSIZE
+#define WSAEINTR            EINTR
+#define WSAEINPROGRESS      EINPROGRESS
+#define WSAEADDRINUSE       EADDRINUSE
+#define WSAENOTSOCK         EBADF
+#define INVALID_SOCKET      (SOCKET)(~0)
+#define SOCKET_ERROR        -1
+#endif
+
+inline int myclosesocket(SOCKET& hSocket)
+{
+    if (hSocket == INVALID_SOCKET)
+        return WSAENOTSOCK;
+#ifdef WIN32
+    int ret = closesocket(hSocket);
+#else
+    int ret = close(hSocket);
+#endif
+    hSocket = INVALID_SOCKET;
+    return ret;
+}
+#define closesocket(s)      myclosesocket(s)
+
+#endif