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/castle.lua

@@ -140,8 +140,7 @@ des.object(object[4],53,11)
 des.object(object[4],54,11)
 des.object(object[4],55,11)
 -- THE WAND OF WISHING in 1 of the 4 towers
-local px, py = place:rndcoord(1);
-des.object({ id = "chest", trapped = 0, locked = 1, x = px, y = py,
+des.object({ id = "chest", trapped = 0, locked = 1, coord = place:rndcoord(1),
              contents = function()
                 des.object("wishing");
              end