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/hello.h

@@ -1,5 +1,5 @@
-#ifndef _HELLO_H
-#define _HELLO_H
+#ifndef _GPXE_HELLO_H
+#define _GPXE_HELLO_H
 
 /** @file
  *