Start to phase out krb5_xfree macro, which just casts its argument to

Start to phase out krb5_xfree macro, which just casts its argument to
char* and calls free.

Replace most uses, outside of the LDAP KDB plugin, which doesn't build
on my test system of the moment because of version dependencies.  Add
one explicit cast to make the change warning-neutral (under gcc 4.0.1
on Mac OS X 10.5.6).

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@21812 dc483132-0cff-0310-8789-dd5450dbe970