BUG/MINOR: proxy: always initialize the trash in show servers state

Actually the cleanup in commit 6ff8143f7 ("BUG/MINOR: proxy: fix
dump_server_state()'s misuse of the trash") allowed to spot that the
trash is never reset when dumping a servers state. I couldn't manage
to make it dump garbage even with large setups but didn't find either
where it's cleared between successive calls while other handlers do
explicitly invoke chunk_reset(), so it seems to happen a bit by luck.

Let's use chunk_printf() here for each turn, it makes things clearer.

This could be backported along with previous patch, especially if any
user reports occasional garbage appearing in the show servers output.

src/proxy.c

@@ -1794,7 +1794,7 @@ static int dump_servers_state(struct stream_interface *si)
 		if (srv->srvrq && srv->srvrq->name)
 			srvrecord = srv->srvrq->name;
 
-		chunk_appendf(&trash,
+		chunk_printf(&trash,
 				"%d %s "
 				"%d %s %s "
 				"%d %d %d %d %ld "