Messages: use WriteAUXHOSTMessage, not WriteExternalMessage

This will cause the messages to come out in the correct order with respect
to host-generated messages.  In particular, mine laying from starbases
appears before mine sweeping from ships that happened later.

WriteExternalMessage would have sent all add-on messages at the very end,
confusing programs that read the messages.

message.c

@@ -106,7 +106,7 @@ void Message_Send(struct Message* m, RaceType_Def to)
 {
     assert(m->Length < sizeof(m->Content));
     m->Content[m->Length] = '\0';
-    WriteExternalMessage(to, m->Content);
+    WriteAUXHOSTMessage(to, m->Content);
 }
 
 void Message_SendTemplate(RaceType_Def to, const char* tpl, const Uns32* args, size_t numArgs)