feat: bind_ip setting for the server sockets

bind_ip in sharedconfig.ini picks the local IPv4 address every server's
RakNet sockets listen on (auth, chat, master, world, the dashboard's
connection to master), separate from external_ip, the address players
are sent. Empty or 0.0.0.0 keeps listening on all interfaces; localhost
means 127.0.0.1. Anything that isn't an IPv4 address stops the server
with an error, and each server logs what it bound to.

Fixes #225

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dNet/dServer.h

@@ -98,6 +98,8 @@ protected:
 	Game::signal_t* mShouldShutdown = nullptr;
 	SocketDescriptor mSocketDescriptor;
 	std::string mIP;
+	// Local address the sockets are bound to (bind_ip), empty for all interfaces
+	std::string mBindAddress;
 	int mPort;
 	int mMaxConnections;
 	unsigned int mZoneID;