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.

test/test_des.lua

@@ -521,4 +521,5 @@ function run_tests()
    des.level_init();
 end
 
+nh.debug_flags({mongen = false, hunger = false, overwrite_stairs = true });
 run_tests();