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.

This tries to catch all cases where grub_cpu_to_XXX was used for constant
expressions (including sizeof).

* grub-core/net/arp.c (grub_net_arp_send_request): Increase network try
interval gradually.
* grub-core/net/icmp6.c (grub_net_icmp6_send_request): Likewise.
* grub-core/net/net.c (grub_net_fs_read_real): Likewise.
* grub-core/net/tftp.c (tftp_open): Likewise.
* include/grub/net.h (GRUB_NET_INTERVAL_ADDITION): New define.

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

	* grub-core/net/bootp.c: Likewise.
	* grub-core/net/dns.c: Likewise.
	* grub-core/net/icmp6.c: Likewise.
	* grub-core/net/net.c: Likewise.

	* include/grub/net.h (grub_net_poll_cards): New argument stop_condition.
	All users updated.
	* grub-core/net/arp.c (have_pending): New var.
	(pending_req): Likewise.
	(grub_net_arp_send_request): Fill pending_req and use have_pending as
	stop indicator.
	(grub_net_arp_receive): Set have_pending.
	* grub-core/net/dns.c (recv_data): New field stop.
	(recv_hook): Set stop.
	(grub_net_dns_lookup): Init stop and use as stop condition.
	* grub-core/net/http.c (http_establish): Use headers_recv as stop
	condition.
	* grub-core/net/net.c (grub_net_poll_cards): New argument
	stop_condition. Stop when it goes true.
	* grub-core/net/tcp.c (grub_net_tcp_open): Use `established' as stop
	indicator.
	* grub-core/net/tftp.c (tftp_open): Use `have_oack' as stop indicator.

	on it.