It's rare with the current 5NM buffer around the origin, but if we use
the hold distance as the buffer like we maybe should it's possible for
the preferred join locations to fall entirely within the home zone. In
that case, fall back to a location within the max-turn-zone that's
outside the home zone and is nearest the IP.
Test cases:
1. Target is not threatened.
The IP should be placed on a direct heading from the origin to the
target at the max ingress distance, or very near the origin airfield
if the airfield is closer to the target than the IP distance.
2. Unthreatened home zone, max IP between origin and target, safe
locations available for IP.
The IP should be placed in LAR at the closest point to home.
3. Unthreatened home zone, origin within LAR, safe locations available
for IP.
The IP should be placed near the origin airfield to prevent
backtracking more than needed.
4. Unthreatened home zone, origin entirely nearer the target than LAR,
safe locations available for IP.
The IP should be placed in LAR as close as possible to the origin.
5. Threatened home zone, safe locations available for IP.
The IP should be placed in LAR as close as possible to the origin.
6. No safe IP.
The IP should be placed in LAR at the point nearest the threat
boundary.
