Put TUN down on client disconnect.

On startup client creates TUN interface in UP state and kept it UP even
if connection to the server was lost. Creating interface in DOWN state,
turning it UP on successful (re-)connection to server and DOWN on either
disconnect or connection loss would enable DHCP client (say dhclient5)
to detect necessity for lease renewal.

Added a client configuration parameter to create TUN interface in DOWN
state and commands to enable, disable, and query the configuration
parameter.
Enabling the parameter causes client to put all unused TUN interfaces
DOWN, create new TUN interfaces in DOWN state, and turn TUN interfaces
corresponding to active sessions DOWN on connection loss or
disconnecting from server.
Disabling the parameter forces client to turn all TUN interfaces UP and
create new TUN interfaces in UP state.
Default value is 'Disable'.

src/Cedar/Client.h

@@ -195,6 +195,7 @@ struct CLIENT_CONFIG
 	UINT KeepConnectProtocol;				// Protocol
 	UINT KeepConnectInterval;				// Interval
 	bool NoChangeWcmNetworkSettingOnWindows8;	// Don't change the WCM network settings on Windows 8
+	bool NicDownOnDisconnect;				// Put NIC down on disconnect/connection loss and put it up again after connecting to VPN server
 };
 
 // Version acquisition