Add a draw_border() function and use it to draw borders.

also `#define`d a border colour (currently c_ltgray, but perhaps should be c_dkgray?) for the possibility of consistency.

grew out of a fix for a border joint in the veh_interact window, so that's there too, as well as some minor comment fixes.

internal borders are still generally c_dkgray, not sure whether they should change, or what should happen there.