[rotorcraft] force MODE_STARTUP instead of KILL until ahrs is aligned

- Go to MODE_STARTUP (which defaults to AP_MODE_KILL) instead of AP_MODE_KILL as long as the ahrs is not aligned.
- Remove AUTOPILOT_DISABLE_AHRS_KILL from all example airframes (but kept the functionality for now).

MODE_STARTUP is set to AP_MODE_NAV for the autonomous (without RC) ARDrone2.
Without AUTOPILOT_DISABLE_AHRS_KILL it will force MODE_STARTUP until the ahrs is aligned,
but still not allow to turn on the motors (as that also checks for ahrs_is_aligned).

As far as I can see this should solve #964 as long as you don't have AUTOPILOT_DISABLE_AHRS_KILL defined.
That is what I realize when looking at the code. I've not run into this case for a long time and didn't payed much attention before, but isn't this a problem ? You can have your RC valid with failsafe mode in case of critical bat level (if enabled) and GPS lost (if not disabled). So can you take control other the plane in this cases ? I think so, but I prefer to check.