runway: Increase maximum heading error to 20 degrees for on-runway
runway: Support distance_start_m = nil and on-short-runway signal
runway: Make remaining distance available to callback of on-short-runway
runway: Make distance_start_m nil by default
runway: Require groundspeed > 40 knots and AGL < 100 feet
runway: Emit remaining-distance signal during RTO
Merge pull request #2 from onox/master

Add on-short-runway signal and remaining-distance signal during RTO