A FR for general ship trail enhancements

Desired new table options
* Boolean controlling if trail is a contrail (present in nebula only) or not (present in all missions)
* Boolean controlling if trail threshold measures speed in absolute terms or relative to a direction of travel.
* Vector normal controlling what direction the speed will be measured in, intended to permit different contrails to appear when flying or drifting in different directions
* Float controlling minimum speed trail appears at.

Contrail speed threshold is currently set on the mission level. I propose treating that as a multiplier, based on the default threshold compared to the mission threshold, and then applying that to contrail type trails only. Non-contrails are desired for engine trails or light streaks and so should not be affected by nebula related settings.

Replaces my previous request of #5274