check is_replay on [set_global_variable] and [do_command]

adds a new method is_replay() to check whether we are in a replay.
fixes http://gna.org/bugs/?21906
Also also we make [do_command]/run_in_synced_context_if_not_already more
robust by checking is_replay

src/game_events/action_wml.cpp

@@ -1139,7 +1139,7 @@ WML_HANDLER_FUNCTION(role, /*event_info*/, cfg)
 /// @todo Finish experimenting.
 WML_HANDLER_FUNCTION(set_global_variable,/**/,pcfg)
 {
-	if (get_replay_source().at_end() || (network::nconnections() != 0))
+	if (!resources::controller->is_replay())
 		verify_and_set_global_variable(pcfg);
 }
 

src/play_controller.hpp

@@ -195,7 +195,7 @@ public:
 	bool is_lingering() { return linger_; }
 
 	class hotkey_handler;
-
+	virtual bool is_replay() { return false; }
 protected:
 
 	game_display& get_display();

src/replay_controller.hpp

@@ -16,6 +16,7 @@
 #ifndef REPLAY_CONTROLLER_H_INCLUDED
 #define REPLAY_CONTROLLER_H_INCLUDED
 
+#include "global.hpp"
 #include "game_end_exceptions.hpp"
 #include "saved_game.hpp"
 #include "play_controller.hpp"
@@ -54,7 +55,8 @@ public:
 	bool recorder_at_end();
 
 	class hotkey_handler;
-
+	
+	virtual bool is_replay() OVERRIDE { return true; }
 protected:
 	virtual void init_gui();
 

src/synced_context.cpp

@@ -103,10 +103,18 @@ bool synced_context::run_in_synced_context_if_not_already(const std::string& com
 	switch(synced_context::get_synced_state())
 	{
 	case(synced_context::UNSYNCED):
+	{
+		if(resources::controller->is_replay())
+		{
+			ERR_REPLAY << "ignored attempt to invoke a synced command during replay\n";
+		}
 		return run_in_synced_context(commandname, data, use_undo, show, true, error_handler);
+	}
 	case(synced_context::LOCAL_CHOICE):
 		ERR_REPLAY << "trying to execute action while being in a local_choice" << std::endl;
 		//we reject it because such actions usually change the gamestate badly which is not intented during a local_choice.
+		//Also we cannot invoke synced commands here, becasue multiple clients might run local choices
+		//simultaniously so it could result in invoking different synced commands simultaniously.
 		return false;
 	case(synced_context::SYNCED):
 	{