resolv: Do not send queries for non-host-names in nss_dns [BZ #24112]

Before this commit, nss_dns would send a query which did not contain a
host name as the query name (such as invalid\032name.example.com) and
then reject the answer in getanswer_r and gaih_getanswer_slice, using
a check based on res_hnok.  With this commit, no query is sent, and a
host-not-found error is returned to NSS without network interaction.