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.cpp

@@ -0,0 +1,61 @@
+// 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.
+
+#include "protocol.h"
+
+CMessageHeader::CMessageHeader()
+{
+    memcpy(pchMessageStart, ::pchMessageStart, sizeof(pchMessageStart));
+    memset(pchCommand, 0, sizeof(pchCommand));
+    pchCommand[1] = 1;
+    nMessageSize = -1;
+    nChecksum = 0;
+}
+
+CMessageHeader::CMessageHeader(const char* pszCommand, unsigned int nMessageSizeIn)
+{
+    memcpy(pchMessageStart, ::pchMessageStart, sizeof(pchMessageStart));
+    strncpy(pchCommand, pszCommand, COMMAND_SIZE);
+    nMessageSize = nMessageSizeIn;
+    nChecksum = 0;
+}
+
+std::string CMessageHeader::GetCommand() const
+{
+    if (pchCommand[COMMAND_SIZE-1] == 0)
+        return std::string(pchCommand, pchCommand + strlen(pchCommand));
+    else
+        return std::string(pchCommand, pchCommand + COMMAND_SIZE);
+}
+
+bool CMessageHeader::IsValid() const
+{
+    // Check start string
+    if (memcmp(pchMessageStart, ::pchMessageStart, sizeof(pchMessageStart)) != 0)
+        return false;
+
+    // Check the command string for errors
+    for (const char* p1 = pchCommand; p1 < pchCommand + COMMAND_SIZE; p1++)
+    {
+        if (*p1 == 0)
+        {
+            // Must be all zeros after the first zero
+            for (; p1 < pchCommand + COMMAND_SIZE; p1++)
+                if (*p1 != 0)
+                    return false;
+        }
+        else if (*p1 < ' ' || *p1 > 0x7E)
+            return false;
+    }
+
+    // Message size
+    if (nMessageSize > MAX_SIZE)
+    {
+        printf("CMessageHeader::IsValid() : (%s, %u bytes) nMessageSize > MAX_SIZE\n", GetCommand().c_str(), nMessageSize);
+        return false;
+    }
+
+    return true;
+}