avconv: move audio_sample_fmt to options context.

Also document it and replace undocumented and inconsistent
'-sample_fmt list' syntax with -sample_fmts.

cmdutils.h

@@ -277,6 +277,12 @@ void show_protocols(void);
  */
 void show_pix_fmts(void);
 
+/**
+ * Print a listing containing all the sample formats supported by the
+ * program.
+ */
+int show_sample_fmts(const char *opt, const char *arg);
+
 /**
  * Return a positive value if a line read from standard input
  * starts with [yY], otherwise return 0.