Refactor: Renamed tent3d_line_extra_list to Teline3ListEx.

It's internal class and the type name is not exposed to outer world,
but it's more comfortable to see things named in the consistent way.
surface: Make F15's gun offset be reflected to HUD's crosshair.

Also it's now customizable from Squirrel initialization script F15.nut.
The real F15's gun is adjusted to point two degrees up from fuselage's
forward direction.