krb5: make krb5_pac a heim_object_t

Make krb5_pac a heim_object_t and use heim_retain() (i.e. reference count +1)
as the copy constructor for the type decoration.

Note: this assumes that PACs included in naming attributes are immutable.

lib/krb5/init_creds_pw.c

@@ -35,7 +35,8 @@
  */
 
 #include "krb5_locl.h"
-#include "../base/heimbasepriv.h" /* XXX */
+
+#include <heimbasepriv.h>
 
 struct pa_info_data {
     krb5_enctype etype;