Header rearrangement.

I want to get to the point where any header in include/ reflects a
standard user-level header (e.g. a POSIX header), while everything that's
specific to gPXE lives in include/gpxe/.  Headers that reflect a Linux
header (e.g. if_ether.h) should also be in include/gpxe/, with the same
name as the Linux header and, preferably, the same names used for the
definitions.

src/include/gpxe/if_arp.h

@@ -1,5 +1,5 @@
-#ifndef	_IF_ARP_H
-#define	_IF_ARP_H
+#ifndef	_GPXE_IF_ARP_H
+#define	_GPXE_IF_ARP_H
 
 /** @file
  *
@@ -97,4 +97,4 @@ static inline void * arp_target_pa ( struct arphdr *arphdr ) {
 	return ( arp_target_ha ( arphdr ) + arphdr->ar_hln );
 }
 
-#endif	/* _IF_ARP_H */
+#endif	/* _GPXE_IF_ARP_H */