examples: use do/while loop for multi examples

It's conceivable that after the first time curl_multi_perform returns,
the outvalue still_running will be 0, but work will have been done. This
is shown by a workload of small, purely file:// based URLs. Ensure that
we always read pending messages off the multi handle by forcing the
while loop to run at least once.

docs/examples/multi-debugcallback.c

@@ -144,7 +144,7 @@ int main(void)
   /* we start some action by calling perform right away */
   curl_multi_perform(multi_handle, &still_running);
 
-  while(still_running) {
+  do {
     struct timeval timeout;
     int rc; /* select() return code */
 
@@ -195,7 +195,7 @@ int main(void)
       curl_multi_perform(multi_handle, &still_running);
       break;
     }
-  }
+  } while(still_running);
 
   curl_multi_cleanup(multi_handle);