regcomp.c: Use more precise definition of folding chars

regcomp.c: Use more precise definition of folding chars

Previously, in the Latin1 range, whether a character is alphabetic or
not has served as a surrogate for if the character participates in
folds, as it is a superset of the folding class, with two characters
which are alpha but not folding: the masculine and feminine ordinal
indicators.  But we have plenty of bits available in the bit array for
Latin1 character classifications, so this commit makes the definition
precise.