Make 'LogContext' a typedef visible throughout the code.

Same principle again - the more of these structures have globally
visible tags (even if the structure contents are still opaque in most
places), the fewer of them I can mistake for each other.

terminal.h

@@ -234,7 +234,7 @@ struct terminal_tag {
 
     void *frontend;
 
-    void *logctx;
+    LogContext *logctx;
 
     struct unicode_data *ucsdata;