Moved if_ether.h and if_arp.h to include/gpxe, for consistency with Linux

kernel.

Removed obsolete struct arprequest from if_arp.h and put it in nic.c so
that nic.c will still compile.  ARP will very shortly be handled by
net/arp.c instead.

src/include/gpxe/if_ether.h

@@ -0,0 +1,31 @@
+#ifndef	_IF_ETHER_H
+#define	_IF_ETHER_H
+
+#include <stdint.h>
+
+#define ETH_ALEN		6	/* Size of Ethernet address */
+#define ETH_HLEN		14	/* Size of ethernet header */
+#define	ETH_ZLEN		60	/* Minimum packet */
+#define	ETH_FRAME_LEN		1514	/* Maximum packet */
+#define ETH_DATA_ALIGN		2	/* Amount needed to align the data after an ethernet header */
+#ifndef	ETH_MAX_MTU
+#define	ETH_MAX_MTU		(ETH_FRAME_LEN-ETH_HLEN)
+#endif
+
+#define ETH_P_IP	0x0800		/* Internet Protocl Packet */
+#define ETH_P_ARP	0x0806		/* Address Resolution Protocol */
+#define ETH_P_RARP	0x8035		/* Reverse Address resolution Protocol */
+#define ETH_P_IPV6	0x86DD		/* IPv6 over blueblook */
+#define ETH_P_SLOW	0x8809		/* Ethernet slow protocols */
+
+/** An Ethernet link-layer header */
+struct ethhdr {
+	/** Destination MAC address */
+        uint8_t h_dest[ETH_ALEN];
+	/** Source MAC address */
+        uint8_t h_source[ETH_ALEN];
+	/** Protocol ID */
+        uint16_t h_protocol;
+} __attribute__ ((packed));
+
+#endif	/* _IF_ETHER_H */