krb5_kt_get_type should return const char *.

krb5_kt_get_type should return const char *.

The code returns a pointer to static structures. Just to enforce the
assumption that users can not change the returned data.

Change prototype of krb5_kt_get_type to return const char *. The other
changes are to clean up warnings - no change in code - usage assumed const.

ticket: new

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@19143 dc483132-0cff-0310-8789-dd5450dbe970