ffmpeg: more expressive sample rate automatic selection.

Output now lists the available sample rates with commands like
ffmpeg -f lavfi -i aevalsrc=0 -ar 20000 -y /tmp/out.mp3

ffmpeg.c

@@ -953,7 +953,18 @@ static void choose_sample_rate(AVStream *st, AVCodec *codec)
             }
         }
         if (best_dist) {
-            av_log(st->codec, AV_LOG_WARNING, "Requested sampling rate unsupported using closest supported (%d)\n", best);
+            int i;
+            const int *sample_rates = codec->supported_samplerates;
+            av_log(st->codec, AV_LOG_WARNING,
+                   "Requested sampling rate (%dHz) unsupported, using %dHz instead\n"
+                   "Available sampling rates for %s:",
+                   st->codec->sample_rate, best, codec->name);
+            for (i = 0; sample_rates[i]; i++) {
+                if (!sample_rates[i + 1]) av_log(st->codec, AV_LOG_WARNING, " and");
+                else if (i)               av_log(st->codec, AV_LOG_WARNING, ",");
+                av_log(st->codec, AV_LOG_WARNING, " %d", sample_rates[i]);
+            }
+            av_log(st->codec, AV_LOG_WARNING, ".\n");
         }
         st->codec->sample_rate = best;
     }