pp.c: Changing case of utf8 strings under locale uses locale for < 255

pp.c: Changing case of utf8 strings under locale uses locale for < 255

As proposed on p5p and approved, this changes the functions uc(), lc(),
ucfirst(), and lcfirst() to respect locale for code points < 255; and
use Unicode semantics for those above 255.  This results in better, but
not perfect results, as noted in the changed pods, and brings these
functions into line with how regular expression pattern matching already
works.