cmdutils: make show_usage() use av_log()

Avoid printing on stdout when show_usage is used in an error message.

avconv.c

@@ -4187,9 +4187,9 @@ static int opt_audio_qscale(OptionsContext *o, const char *opt, const char *arg)
 
 static void show_usage(void)
 {
-    printf("Hyper fast Audio and Video encoder\n");
-    printf("usage: %s [options] [[infile options] -i infile]... {[outfile options] outfile}...\n", program_name);
-    printf("\n");
+    av_log(NULL, AV_LOG_INFO, "Hyper fast Audio and Video encoder\n");
+    av_log(NULL, AV_LOG_INFO, "usage: %s [options] [[infile options] -i infile]... {[outfile options] outfile}...\n", program_name);
+    av_log(NULL, AV_LOG_INFO, "\n");
 }
 
 static int opt_help(const char *opt, const char *arg)

ffmpeg.c

@@ -4593,9 +4593,9 @@ static int opt_audio_qscale(OptionsContext *o, const char *opt, const char *arg)
 
 static void show_usage(void)
 {
-    printf("Hyper fast Audio and Video encoder\n");
-    printf("usage: %s [options] [[infile options] -i infile]... {[outfile options] outfile}...\n", program_name);
-    printf("\n");
+    av_log(NULL, AV_LOG_INFO, "Hyper fast Audio and Video encoder\n");
+    av_log(NULL, AV_LOG_INFO, "usage: %s [options] [[infile options] -i infile]... {[outfile options] outfile}...\n", program_name);
+    av_log(NULL, AV_LOG_INFO, "\n");
 }
 
 static int opt_help(const char *opt, const char *arg)

ffplay.c

@@ -3105,9 +3105,9 @@ static const OptionDef options[] = {
 
 static void show_usage(void)
 {
-    printf("Simple media player\n");
-    printf("usage: %s [options] input_file\n", program_name);
-    printf("\n");
+    av_log(NULL, AV_LOG_INFO, "Simple media player\n");
+    av_log(NULL, AV_LOG_INFO, "usage: %s [options] input_file\n", program_name);
+    av_log(NULL, AV_LOG_INFO, "\n");
 }
 
 static int opt_help(const char *opt, const char *arg)

ffprobe.c

@@ -1439,9 +1439,9 @@ end:
 
 static void show_usage(void)
 {
-    printf("Simple multimedia streams analyzer\n");
-    printf("usage: %s [OPTIONS] [INPUT_FILE]\n", program_name);
-    printf("\n");
+    av_log(NULL, AV_LOG_INFO, "Simple multimedia streams analyzer\n");
+    av_log(NULL, AV_LOG_INFO, "usage: %s [OPTIONS] [INPUT_FILE]\n", program_name);
+    av_log(NULL, AV_LOG_INFO, "\n");
 }
 
 static int opt_format(const char *opt, const char *arg)