Taint more operands with case changes

Taint more operands with case changes

 The documentation says that Perl taints certain operations when subject
 to locale rules, such as lc() and ucfirst().  Prior to this commit
 there were exceptions when the operand to these functions contained no
 characters whose case change actually varied depending on the locale,
 for example the empty string or above-Latin1 code points.  Changing to
 conform to the documentation simplifies the core code, and yields more
 consistent results.