Get rid of casts of free() argument to char*, except where it's

Get rid of casts of free() argument to char*, except where it's
casting away const (so as to make this change warning-neutral), and in
unicode source (which we may want to keep in sync with another
source), and krb5_xfree macro (to be handled separately).

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@21870 dc483132-0cff-0310-8789-dd5450dbe970