Allow network activity to be temporarily suspended.

Added the function SetNetworkActive() which when called with argument set to false disconnects all nodes and sets the flag fNetworkActive to false. As long as this flag is false no new connections are attempted and no incoming connections are accepted. Network activity is reenabled by calling the function with argument true.
Add RPC command "togglenetwork" to toggle network activity temporarily.
Add gui feedback and control of network activity state.

Add getNetworkActive()/setNetworkActive() method to client model.
Send network active status along with NotifyNumConnectionsChanged.
Indicate in tool tip of gui status bar network indicator whether network activity is disabled.
Indicate in debug window whether network activity is disabled and add button to allow user to toggle network activity state.