tests/cavern.tst: Run diffpos <expected> <actual> so the reports of
'Added' and 'Deleted' stations are the more natural way round.
src/cad3d.c: Check for errors from img_rewind() and report them.
src/dump3d.c: Report the data style of legs.
doc/TODO.htm: The data style of legs is now stored in the 3d file,
so remove "flag nosurvey legs in .3d file" from todo list.
doc/3dformat.htm,src/img.c: More work toward v8 of 3d format.