Clean up libdisk for WARNS=4

I did a bad commit for WARNS=4 of libdisk which breaks non-i386
build.  I'm trying to solve this in other archs, too.

http://www.imgsrc.co.jp/~kuriyama/BSD/libdisk-20040404.diff

o Added many __unused's to avoid warnings on all archs.
o Added file local struct definition in open_ia64_disk.c to avoid
  pointer alignment problem.
o Renamed some variables to resolve conflicts with other enum
  constants (same as in i386).
o Used uint64_t buffer instead of u_char buffer.  This simplifed
  uint64_t array manipulation in latter of this function.  But it
  looks ugly.
o Used ssize_t storage instead of direct comparison of
  singed/unsigned.


I'm not sure all of this is done by good way or not.  So I'd like to
be reviewed...


-- 
Jun Kuriyama <kuriyama_at_imgsrc.co.jp> // IMG SRC, Inc.
             <kuriyama_at_FreeBSD.org> // FreeBSD Project