Setting the cmdlist pointer in the bind-key to NULL to prevent it being freed

after the command is executing is bogus because it may still be needed if the
same command is going to be executed again (for example if you "bind-key a
bind-key b ..."). Making a copy is hard, so instead add a reference count to
the cmd_list.

While here, also print bind-key -n and the rest of the flags properly.

Fixes problem reported by mcbride@.

tmux.h

@@ -1161,7 +1161,10 @@ struct cmd {
 
 	TAILQ_ENTRY(cmd) qentry;
 };
-TAILQ_HEAD(cmd_list, cmd);
+struct cmd_list {
+	int		 references;
+	TAILQ_HEAD(, cmd) list;
+};
 
 struct cmd_entry {
 	const char	*name;