exportlegends includes new maps

I modified `exportlegends.lua` to also export the new maps (`Nobility and Holdings`, and `Diplomacy`).  

I also found but couldn't work out how to fix a bug - regardless of whether the argument was `maps` or `all`, the maps were exporter but the other information was not (legends XML, text files, tile map.  
I've renamed this to `sel_idx` in https://github.com/DFHack/df-structures/pull/16 - using anon_* names isn't safe, since their names will change if the structure changes or fields before it are identified.