codecs: avoid having to print error message

The mp_select_decoders() function doesn't have a log context (and I
don't want to give it one), so get rid of the mp_msg error message by
enhancing the semantics such that the syntax error is replaced by a new
feature. Now doing "--ad=something" will enable all decoders in the
"something" module, same as "--ad=something:*". Pretty useless, but gets
rid of the annoyance.

common/codecs.c

@@ -98,9 +98,8 @@ struct mp_decoder_list *mp_select_decoders(struct mp_decoder_list *all,
         struct mp_decoder_list *dest = exclude ? remove : list;
         bstr family, decoder;
         if (!bstr_split_tok(entry, ":", &family, &decoder)) {
-            mp_msg(MSGT_CFGPARSER, MSGL_ERR, "Decoders must be specified as "
-                   "'family:decoder' for the --ad/--vd options.\n");
-            break;
+            family = entry;
+            decoder = bstr0("*");
         }
         if (bstr_equals0(decoder, "*")) {
             for (int n = 0; n < all->num_entries; n++) {