Lua pushkey and getlin

Allow lua nh.pushkey to push multiple keys,
make getlin return the keys in the command queue.

test/test_src.lua

@@ -121,3 +121,13 @@ for func, fval in pairs(tests) do
       end
    end
 end
+
+function test_getlin()
+   nh.pushkey("AbC");
+   local str = nh.getlin("What?");
+   if str ~= "AbC" then
+      error("nh.getlin fail, got \"" .. str .. "\"");
+   end
+end
+
+test_getlin();