Feature #7756: Allow server to supply a reason to kicked/banned clients

This commit adds the missing feature of allowing the server owner to
    provide a reason for kicking/banning a client, which the client sees in
    a pop-up window after being kicked. The implementation extends the
    network protocol by adding a new network action called
    NETWORK_ACTION_KICKED that is capable of having an error string, unlike
    the other network error packages.  Additionally, the kick function
    broadcasts a message to all clients about the kicked client and the
    reason for the kick.

src/network/network_server.h

@@ -89,7 +89,7 @@ public:
 	NetworkRecvStatus SendMove(ClientID client_id, CompanyID company_id);
 
 	NetworkRecvStatus SendClientInfo(NetworkClientInfo *ci);
-	NetworkRecvStatus SendError(NetworkErrorCode error);
+	NetworkRecvStatus SendError(NetworkErrorCode error, const char *reason = nullptr);
 	NetworkRecvStatus SendChat(NetworkAction action, ClientID client_id, bool self_send, const char *msg, int64 data);
 	NetworkRecvStatus SendJoin(ClientID client_id);
 	NetworkRecvStatus SendFrame();