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/errno.h

@@ -125,10 +125,11 @@
 #define ENOENT		0xd4
 #define EAFNOSUPPORT	0xd5
 #define EAGAIN		0xd6
+#define EIO		0xd7
 
 /* Data structures and declarations */
 
-#include "tables.h"
+#include <gpxe/tables.h>
 
 extern int errno;