utf8.c: Allow Changed behavior of utf8 under locale

utf8.c: Allow Changed behavior of utf8 under locale

This changes the 4 case changing functions to take extra parameters to
specify if the utf8 string is to be processed under locale rules when
the code points are < 256.  The current functions are changed to macros
that call the new versions so that current behavior is unchanged.

An additional, static, function is created that makes sure that the
255/256 boundary is not crossed during the case change.