nss_dns: Split getanswer_ptr from getanswer_r

And expand the use of name_ok and qtype in getanswer_ptr (the
former also in getanswer_r).

After further cleanups, not much code will be shared between the
two functions.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit 0dcc43e9)