This restrict ARP handling to MAC and IP addresses but in practice we need
only this case anyway and other cases are very rar if exist at all. It makes
code much simpler and less error-prone.

	All users updated.
	(free_rsm): Free header as well.
	(free_old_fragments): Fix memory leak.
	* grub-core/net/netbuff.c (grub_netbuff_free): Make return void.
	* grub-core/net/tftp.c (tftp_receive): Fix memory leak.
	(destroy_pq): Likewise.
	* include/grub/net/netbuff.h (grub_netbuff_free): Make return void.

	on it.