Lua: coordinate tweaking

Make selection rndcoord return a table with x and y keys.
Allow (most) coordinate parameters accept such a table.
Fix selection and des lua tests broken by the above changes and
an earlier change, because selections tried to set terrain
at column 0, and it now causes a complaint.

dat/Bar-strt.lua

@@ -95,6 +95,5 @@ des.monster("giant eel", 39, 15)
 -- Monsters on siege duty.
 local ogrelocs = selection.floodfill(37,7) & selection.area(40,03, 45,20)
 for i = 0, 11 do
-   local x,y = ogrelocs:rndcoord(1);
-   des.monster({ id = "ogre", coord={x,y}, peaceful=0 })
+   des.monster({ id = "ogre", coord = ogrelocs:rndcoord(1), peaceful = 0 })
 end