cmdutils: refactor -codecs option.

Make it print a list of AVCodecDescriptors.

Add new -decoders and -encoders options that print lists of decoders and
encoders respectively.

cmdutils.h

@@ -278,6 +278,18 @@ void show_formats(void);
  */
 void show_codecs(void);
 
+/**
+ * Print a listing containing all the decoders supported by the
+ * program.
+ */
+int show_decoders(const char *opt, const char *arg);
+
+/**
+ * Print a listing containing all the encoders supported by the
+ * program.
+ */
+int show_encoders(const char *opt, const char *arg);
+
 /**
  * Print a listing containing all the filters supported by the
  * program.