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.

unix/uxcons.c

@@ -405,7 +405,7 @@ void old_keyfile_warning(void)
     postmsg(&cf);
 }
 
-void console_provide_logctx(void *logctx)
+void console_provide_logctx(LogContext *logctx)
 {
     console_logctx = logctx;
 }