* dnParent - dn's parent, in-place
* note: the incoming dn is assumed to be normalized/prettyfied,
* so that escaped rdn/ava separators are in '\'+hexpair form
+ *
+ * note: "dn" and "pdn" can point to the same berval;
+ * beware that, in this case, the pointer to the original buffer
+ * will get lost.
*/
void
dnParent(