Simplify the ui-out progress API

I noticed that 'progress' is a method on ui-out, but it seems to me
that it would be better if the only API were via the progress_meter
class.  This patch makes this change, changing progress to be a method
on the meter itself.

gdb/debuginfod-support.c

@@ -129,7 +129,7 @@ progressfn (debuginfod_client *c, long cur, long total)
       data->meter.emplace (current_uiout, message, 1);
     }
 
-  current_uiout->progress ((double)cur / (double)total);
+  data->meter->progress ((double)cur / (double)total);
 
   return 0;
 }