codepage: strip accents during shortname upper-casing

codepage: strip accents during shortname upper-casing

If we don't have a direct upper-case equivalent to a character, but we
*do* have the "naked" version of the upper-case character (defined as
the first character of the canonical Unicode decomposition of the
case-mapped character), then use it for the intracodepage table (used
for shortnames.)  This matches DOS behaviour.  No obvious way, yet, to
handle the fact that DOS doesn't uppercase the lowercase sigma, but
that's not a huge problem; besides, lowercase sigma is mapped to
position 0xE5, which would require dealing with the special handling
of this character in the first position.