Log which config file we read

It is often not entirely clear what options Tor was built with, so it
might not be immediately obvious which config file Tor is using when it
found one. Log the config file at startup.

src/or/config.c

@@ -4028,6 +4028,8 @@ load_torrc_from_disk(int argc, char **argv)
           "Unable to open configuration file \"%s\".", fname);
       goto err;
     }
+  } else {
+    log(LOG_NOTICE, LD_CONFIG, "Read configuration file \"%s\".", fname);
   }
 
   return cf;