hurd/version.h

I've added an installed header file called hurd/version.h, which I will
commit when I commit the 64-bit changes.  Here are its contents now:

/* This file just gives a value that can be tested easily with #if as a
   gross check of the Hurd interface version.  It has the format YYYYMMDD
   and will only ever be increased.  This will be bumped whenever either
   the RPC interfaces or the library APIs change.  */

#define HURD_INTERFACE_VERSION	     20020609


We could define a couple different ones but it seems to safe to just have
one thing that gets bumped arbitrarily frequently.  I will use this in libc
for an autoconf check to barf when the headers are pre-64 bit.  Other
packages could use it to check either in autoconf or just #if to notice
miscellaneous changes in the API of libstore et al.