assistedcontrol: braking optimisation

1. vtolsettings: introduced PIDs for braking, as well as velocity feedforward parameter to tune
2. pathstatus: we now capture the brake exit reason code and the distance to the intended stopping point when we exit braking and enter hold
3. pathfollower now uses dedicated braking PIDs.  BRaking PIDs have zero I term.
4. pathfollower captures the brake exit reason code and distance error
5. plans.c: fixed bug where we failed to set starting velocity which would trigger entry to hold immediately
6. plans.c: in transitioning from brake to hold, both modes target the same final end position. Given that braking exits at 90% of velocity reduction, the final travel should have PH pull the quad into place.