"Strict" Flight Mode

Add a configuration file option (defaults to enabled) to use "strict" flight mode.

The concept is that only Flight Mode should change WWAN state, and WWAN state needs to follow flight mode state.

It's not convenient/possible to improve this by returning errors when a call is made to set a WWAN device blocked since these calls can also happen when a WWAN type rfkill switch is triggered in the kernel. Similarly, flight mode itself uses the same calls.

It should be up to UI to not expose or use the explicitly WWAN device toggle, and only show flight mode.
By continuing here, you skip the if (!ret) failure check below ( which was added recently as well ) which means that the phone could go into flight-mode with the modem still on and the user wouldn't know.