Fix vehicle::solar_epower() (fixes #10086)

Looking at `vehicle::solar_epower()` my logic says it should work as intended, i.e.:

1. If the part is indoors don't add power (in my logic all underground terrain where one could put a solar panel is by definition indoors)
2. If it's not indoors add the power based on the ground-level natural light level (as opposed to the natural light level of where the player actually is right now)

I bet my assumption of TFLAG_INDOORS being present for all underground terrain is false, but I believe it would be logical (or are we distinguishing underground between natural and man-made structures? e.g. cave vs. LMOE or a lab).

In any case my feeling is that right now this should at least fix the "spending time underground will prevent aboveground solars from charging, despite them being processed" part of the problem.

Please comment.