Add additional codepage data from the Unicode Consortium.
Additionally, add support for generating codepage files for the case
where the filesystem codepage and the display codepage do not match.
The standard Makefile doesn't actually do that, however.

Make the codepage data all derive from the Unicode tables, and create
files that could be dynamically loaded in the future.

Initial support for VFAT long filenames; currently hard-coded to
codepage 865, no support for other codepages or for accessing files
with names which aren't in the current codepage.  This hopefully
shouldn't be an issue for SYSLINUX applications.

Add support for creating codepage/Unicode mapping tables; this is a
step toward VFAT long filename support.