OpenVPN: Add packet scrambling/obfuscation feature

This allows an OpenVPN client to bypass a firewall which is aware of the protocol and is able to block it.
The XOR mask set on the server has to be the same on the client, otherwise it will not be able to connect with certain obfuscation modes.
A special OpenVPN client built with the "XOR patch" is required in order to use this function, because it has never been merged in the official OpenVPN repository.

Two parameters are added to the server configuration: "OpenVPNObfuscationMethod" and "OpenVPNObfuscationMask".
Their value can be retrieved with "OpenVpnObfuscationGet" and set with "OpenVpnObfuscationEnable" in the VPN Command Line Management Utility.

src/Cedar/Proto_OpenVPN.h

@@ -185,6 +185,12 @@
 #define	OPENVPN_MODE_L2							1		// TAP (Ethernet)
 #define	OPENVPN_MODE_L3							2		// TUN (IP)
 
+// Scramble mode
+#define	OPENVPN_SCRAMBLE_MODE_DISABLED			0		// No scramble
+#define	OPENVPN_SCRAMBLE_MODE_XORMASK			1		// XOR the bytes with the specified string
+#define	OPENVPN_SCRAMBLE_MODE_XORPTRPOS			2		// XOR each byte with its position in the buffer
+#define	OPENVPN_SCRAMBLE_MODE_REVERSE			3		// Reverses bytes order, keeping the first byte unchanged
+#define	OPENVPN_SCRAMBLE_MODE_OBFUSCATE			4		// Performs the above steps using the specified string for xormask
 
 //// Type
 
@@ -271,6 +277,7 @@ struct OPENVPN_SESSION
 	OPENVPN_CHANNEL *Channels[OPENVPN_NUM_CHANNELS];	// Channels (up to 8)
 	UINT LastCreatedChannelIndex;						// Channel number that is created in the last
 	UINT Mode;											// Mode (L3 or L2)
+	UINT ObfuscationMode;								// Packet obfuscation/scrambling mode
 	UINT LinkMtu;										// link-mtu
 	UINT TunMtu;										// tun-mtu
 	IPC_ASYNC *IpcAsync;								// Asynchronous IPC connection