Don't allow remote servers to write to our MD client objects by default.

Modules can still opt-in via mreq.remote_write=1 to allow it for
certain moddata.
For example, k4be may want to do this for his geoip-base module which
allows a single server to set moddata "geoip" for all connecting clients,
including remote clients.
If you are a moddata provider then you can enable it like this:
 ModDataInfo mreq;
 [..]
 #if UNREAL_VERSION_TIME >= 202125
 mreq.remote_write = 1;
 #endif
 [..]

See discussion on https://github.com/unrealircd/unrealircd/pull/142

src/api-moddata.c

@@ -80,6 +80,7 @@ moddataadd_isok:
 	m->serialize = req.serialize;
 	m->unserialize = req.unserialize;
 	m->sync = req.sync;
+	m->remote_write = req.remote_write;
 	m->owner = module;
 	
 	if (new_struct)