Mitigate formspec exploits by verifying that the formspec was shown to the user by the server. (#6878)

This doesn't check the fields in anyway whatsoever so it should only be seen as a way to mitigate exploits, a last line of defense to make it harder to exploit bugs in mods, not as a reason to not do all the usually checks.

src/server.h

@@ -591,6 +591,8 @@ private:
 	*/
 	std::queue<con::PeerChange> m_peer_change_queue;
 
+	std::unordered_map<session_t, std::string> m_formspec_state_data;
+
 	/*
 		Random stuff
 	*/