regen/unicode_constants.pl: Convert '-' in names to '_'

Unicode character names can have dashes in them.  These aren't accepted
in C macro names.  Change so both blanks and the hyphen-minus are
converted to underscores.
regen/unicode_constants.pl: Pass through input comments

The data can now have comments, which are converted to C and passed
through
regen/unicode_constants.pl: Remove unused constant

This was added in the 5.17 series, so can't be yet in the field; and
isn't needed.
regen/unicode_constants.pl: Make portable to non-ASCII

This now uses the U+ notation to indicate code points, which is
unambiguous not matter what the platform's character set is.  (charnames
accepts the U+ notation)
regen/unicode_constants.pl: Change #define name

This was added in the 5.17 series so there's no code relying on its
current name.  I think that the abbreviation is clearer.