updated failsafe behavior, fixed arming bug

failsafe now has the following behavior:
- if throttle is less than thr_min, set throttle to 0
- if throttle is greater than throttle min and in manual mode, set
throttle to 0
- if throttle is greater than thr_min and in alt hold mode less than 1
m off the ground, set throttle to 0
- if throttle is greater than thr_min and in alt hold mode and more
than 1 m off ground, head towards ground for 5 seconds and then set
throttle to min

Working but there is a mystery with why failsafe_start_time is not
being reset as expected