Recently, some distributors have set "utf8" to NLS_DEFAULT, therefore,
FAT uses the "iocharset=utf8" as default.  But, since "iocharset=utf8"
doesn't provide the function (lower <-> upper conversion) which FAT
needs, so FAT can't provide suitable behavior.

This patch does:

     - doesn't recognize "utf8" as "iocharset"
     - doesn't use NLS_DEFAULT as default "iocharset"
     - instead of NLS_DEFAULT, adds FAT_DEFAULT_CODEPAGE and
       FAT_DEFAULT_IOCHARSET
   
NOTE: the following looks like buggy, so it's not recommended

    "codepage=437,iocharset=iso8859-1,utf8"

however, some utf8 file name can handle. (in this case, it uses the
table of iso8859-1 for lower <-> upper conversion)
Sign-off-by: Jesse Barnes <jbarnes@engr.sgi.com>
Sign-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Sign-off-by: Linus Torvalds <torvalds@osdl.org>