add [unsynced] actionwml

This can be used like in the following example (note that without the
[unsynced] it would cause OOS becasue rand= might or might not be
called.):

[unsynced]
[store_side]
variable = "side"
side = "$side_number"
[/store_side]
[if]
[variable]
name = "side.controller"
equals = "human"
[/variable]
[then]
[set_variable]
name = "num"
rand = _ "two,four,seven"
[/set_variable]
[message]
message = _ "I want $num stones"
[/message]
[/then]
[/if]
[unsynced]

data/lua/wml-tags.lua

@@ -1505,3 +1505,9 @@ function wml_actions.role(cfg)
 
 	-- no matching unit found, fail silently
 end
+
+function wml_actions.unsynced(cfg)
+	wesnoth.unsynced(function ()
+		wml_actions.command(cfg)
+	end)
+end

src/scripting/game_lua_kernel.cpp

@@ -2937,6 +2937,19 @@ int game_lua_kernel::intf_synchronize_choice(lua_State *L)
 	return 1;
 }
 
+/**
+ * Calls a function in an unsynced context (this specially means that all random calls used by that function will be unsynced).
+ * This is usualy used together with an unsynced if like 'if controller != network'
+ * - Arg 1: function that will be called during the unsynced context.
+ */
+static int intf_do_unsynced(lua_State *L)
+{
+	set_scontext_unsynced sync;
+	lua_pushvalue(L, 1);
+	luaW_pcall(L, 0, 0, false);
+	return 0;
+}
+
 /**
  * Gets all the locations matching a given filter.
  * - Arg 1: WML table.
@@ -4075,6 +4088,7 @@ game_lua_kernel::game_lua_kernel(CVideo * video, game_state & gs, play_controlle
 		{ "unit_defense",             &intf_unit_defense             },
 		{ "unit_movement_cost",       &intf_unit_movement_cost       },
 		{ "unit_resistance",          &intf_unit_resistance          },
+		{ "unsynced",                 &intf_do_unsynced              },
 		{ "add_event_handler",         &dispatch<&game_lua_kernel::intf_add_event                  >        },
 		{ "add_tile_overlay",          &dispatch<&game_lua_kernel::intf_add_tile_overlay           >        },
 		{ "add_time_area",             &dispatch<&game_lua_kernel::intf_add_time_area              >        },