Set ctrl_interface to non-empty value in INTERFACE_ADD to avoid
ctrl_iface.c assuming the control interface is not to be enabled. This
fixes the dynamically added interface to use control interface.
"Add interface" command in File menu can now be used to add a new
network interface to running wpa_supplicant (using INTERFACE_ADD control
interface command). In addition, the network interface is added into
Windows registry (with skip_on_error) for future use. This functionality
is currently enabled only for Windows builds. The user is also prompted
about the possibility of adding an interface if no interfaces are
enabled. This makes it easier to get started without having to touch
registry manually.
