RFC.. defining __rangeof() in cdefs.h

I'm considdereing adding:
Index: sys/cdefs.h
===================================================================
RCS file: /home/ncvs/src/sys/sys/cdefs.h,v
retrieving revision 1.83
diff -u -r1.83 cdefs.h
--- sys/cdefs.h 28 Jul 2004 07:03:42 -0000      1.83
+++ sys/cdefs.h 9 Aug 2004 21:36:41 -0000
_at__at_ -241,6 +241,8 _at__at_
  * require it.
  */
 #define        __offsetof(type, field) ((size_t)(&((type *)0)->field))
+#define        __rangeof(type, start, end) \
+       (__offsetof(type, end) - __offsetof(type, start))
 
 /*
  * Compiler-dependent macros to declare that functions take printf-like


it is used in several places. most importantly in fork1()

and it is defined in several files (*).. we should probably just have 
one copy...


(*) in the form RANGEOF() but if we define it in cdefs.h I'd change that to
__rangeof() to match __offsetof()