cmdutils: add -layouts option.

Extract of the output:

| Individual channels:
| NAME        DESCRIPTION
| FL          front left
| FR          front right
| <snip>
| SDR         surround direct right
|
| Standard channel layouts:
| NAME        DECOMPOSITION
| mono        FC
| stereo      FL+FR
| <snip>
| octagonal   FL+FR+FC+BL+BR+BC+SL+SR
| downmix     DL+DR

cmdutils.h

@@ -359,6 +359,13 @@ int show_protocols(const char *opt, const char *arg);
  */
 int show_pix_fmts(const char *opt, const char *arg);
 
+/**
+ * Print a listing containing all the standard channel layouts supported by
+ * the program.
+ * This option processing function does not utilize the arguments.
+ */
+int show_layouts(const char *opt, const char *arg);
+
 /**
  * Print a listing containing all the sample formats supported by the
  * program.