modify dns_qp_findname_ancestor() to return found name

modify dns_qp_findname_ancestor() to return found name

add a 'foundname' parameter to dns_qp_findname_ancestor(),
and use it to set the found name in dns_nametree.

this required adding a dns_qpkey_toname() function; that was
done by moving qp_test_keytoname() from the test library to qp.c.
added some more test cases and fixed bugs with the handling of
relative and empty names.