Instead of setting up the default keys by building the key struct

directly with a helper function in the cmd_entry, include a table of
bind-key commands and pass them through the command parser and a
temporary cmd_q.

As well as being smaller, this will allow default bindings to be command
sequences which will probably be needed soon.

tmux.h

@@ -1492,7 +1492,6 @@ struct cmd_entry {
 #define CMD_READONLY 0x4
 	int		 flags;
 
-	void		 (*key_binding)(struct cmd *, int);
 	enum cmd_retval	 (*exec)(struct cmd *, struct cmd_q *);
 };