Add EnableInterface and DisableInterface definitions to netctl-front, and a new
wireless.State, InterfaceDisabled, and corresponding entry in
the WirelessState enum in api/netctl.proto so that this new state can be
reported.

The implementations for Manager.Enable/DisableInterface will be added in a later
commit.

For now, do not add any extra handling of calls to Manager when the
interface is disabled; we will let wpa_supplicant propagate errors.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Signed-off-by: girouxj <girouxj@ainfosec.com>