Start moving protocol-specific code to protocol.[ch]pp

Move CMessageHeader from net.h to protocol.[ch]pp, with the
implementation in the .cpp compilation unit (compiling once is enough).

This commit does *not* and should not modify *any* code, it only moves
it from net.h and splits it across protocol.cpp and protocol.hpp.

Indentation changes aside the closest thing to a modification of code is
the addition of the 'TODO' comment (the execution of which requires code
modifications and thus doesn't belong in this commit).

Signed-off-by: Giel van Schijndel <me@mortis.eu>

src/protocol.h

@@ -0,0 +1,52 @@
+// 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 __cplusplus
+# error This header can only be compiled as C++.
+#endif
+
+#ifndef __INCLUDED_PROTOCOL_H__
+#define __INCLUDED_PROTOCOL_H__
+
+#include "serialize.h"
+#include <string>
+
+//
+// Message header
+//  (4) message start
+//  (12) command
+//  (4) size
+//  (4) checksum
+
+extern unsigned char pchMessageStart[4];
+
+class CMessageHeader
+{
+    public:
+        CMessageHeader();
+        CMessageHeader(const char* pszCommand, unsigned int nMessageSizeIn);
+
+        std::string GetCommand() const;
+        bool IsValid() const;
+
+        IMPLEMENT_SERIALIZE
+            (
+             READWRITE(FLATDATA(pchMessageStart));
+             READWRITE(FLATDATA(pchCommand));
+             READWRITE(nMessageSize);
+             if (nVersion >= 209)
+             READWRITE(nChecksum);
+            )
+
+    // TODO: make private (improves encapsulation)
+    public:
+        enum { COMMAND_SIZE=12 };
+        char pchMessageStart[sizeof(::pchMessageStart)];
+        char pchCommand[COMMAND_SIZE];
+        unsigned int nMessageSize;
+        unsigned int nChecksum;
+};
+
+#endif // __INCLUDED_PROTOCOL_H__