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/base/heimbasepriv.h

@@ -65,6 +65,7 @@ enum {
     HEIM_TID_DATA = 134,
     HEIM_TID_DB = 135,
     HEIM_TID_PA_AUTH_MECH = 136,
+    HEIM_TID_PAC = 137,
     HEIM_TID_USER = 255
 
 };