"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.
This does not solve the problem at all. First, urf_arbitrator_set_block() always calls urf_config_set_persist_state() for oFono devices (I don't know if that will still be the case when Tony's changes get merged), so there is no point in calling urf_config_set_persist_state() again. Second, it still can happen that you save WWAN soft=TRUE but fail when setting off the next device, then you fail to enter flight mode *and* have bad values in the "saved-states" file (ALL is FALSE, but WWAN is TRUE). We have again the bug.

To solve this, we need to make FM transactional transactional, so what needs to be done is:

1.- Do not call urf_config_set_persist_state() inside the loop (remove call inside urf_arbitrator_set_block(), remove also from this change).
2.- If and only if the loop has finished and all devices have been put into the desired state, do the calls to urf_config_set_persist_state() for each type.
3.- Return true in the function and urf_daemon_flight_mode() will also save the right state for FM.