Failsafe: make the safety margin 100 us

This makes the valid window for throttle 100 µs but keeps it the same for the other channels.  That allows people with a Frksy transmitter to get reliable failsafe.  In addition the flight mode channel checking is cleaned up (from stac) and the input range on that is also checked when it is configured.  This provides another means to trigger failsafe by setting a narrow window on the flight mode switch and then flicking it out of range (i.e. fixes #596 indirectly).  This has the additional benefit of allowing people to put it on the flight mode switch while using the normal path through the code rather than a hack.