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.

Many routers have long router advertisment interval configured by
default. The Neighbor Discovery protocol (RFC4861) has defined default
MaxRtrAdvInterval value as 600 seconds and
MinRtrAdvInterval as 0.33*MaxRtrAdvInterval. This makes
net_ipv6_autoconf fails more often than not as currently it passively
listens the RA message to perfom address autoconfiguration.

This patch tries to send router solicitation to overcome the problem of
long RA interval.

v2:
use cpu_to_be macro for network byte order conversion
add missing error handling

	Just "packed" doesn't always pack the way we expect.

	* grub-core/net/http.c (parse_line): Handle response 206.
	(http_receive): Stall if too many packets are in the queue.
	(http_establish): Fix range header.
	(http_seek): Fix double free.
	(http_close): Likewise.
	(http_packets_pulled): New function.
	(grub_http_protocol): Set http_seek
	* grub-core/net/tcp.c (grub_net_tcp_socket): New field `i_stall'.
	(ack_real): Set window depending on i_stall.
	(grub_net_send_tcp_packet): Likewise.
	(grub_net_tcp_stall): New function.
	(grub_net_tcp_unstall): Likewise.
	* include/grub/net/tcp.h (grub_net_tcp_stall): New proto.
	(grub_net_tcp_unstall): Likewise.

	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.

Change ofnet.c to disknet.c and remove almost all ieee1275 specific code.
Create grub_net_malloc to handle iee1275 memory issues in a temporary solution.

implement ntoa ipv4 function.

* include/grub/net/arp.h: added arp header, arp cache entry and related constants and functions
* net/arp.c: added functions arp_init_table, arp_find_entry, arp_resolve and arp_receive
* net/ethernet.c (send_ethernet_packet): replaced hardcoded hardware address by parameter target_addr
* net/ethernet.c (recv_ethernet_packet): added call to arp_receive when packet is of type 0x803 (ARP) and only return when packet is of type determined by parameter ethertype
* net/ip.c (send_ip_packet): added call to arp_resolve to determine hardware address of destination
* net/netbuff.c (grub_netbuff_alloc): fixed swapped parameters in call to grub_memalign

Changed the protocols structs to use one struct for each layer.