main: compat message for '-A'

Semantics of '-A' parameters has changed from audio receiver address
(distinct than video) to audio codec. It is, however, doubful if someone
uses it, but just in case.

This compat message could (and should) be deleted later.

src/main.cpp

@@ -948,6 +948,12 @@ static int parse_options(int argc, char *argv[], struct ug_options *opt) {
                         }
                         opt->audio.codec_cfg = optarg;
                         if (!check_audio_codec(optarg)) {
+                                LOG(LOG_LEVEL_WARNING)
+                                    << MOD_NAME
+                                    << "The original semantics of '-A' "
+                                       "parameter has changed,\nplease use "
+                                       "'--audio-host' to specify audio "
+                                       "receiver address.\n";
                                 return -EXIT_FAIL_USAGE;
                         }
                         break;