exportlegends can do site maps, IDd field

Expanded exportlegends.lua to handle site maps as well, and improved
precision of args.  Identified `vs.anon_21`; I'm not sure what the
process is so I've used it here and noted it's discovery.
Use name of now-identified field

Ready for DFHack 40.15-r1 and later
Merge pull request #343 from PeridexisErrant/exportlegends-sites

exportlegends can do site maps, IDd field