nss_dns: Rewrite _nss_dns_gethostbyname4_r using current interfaces

Introduce struct alloc_buffer to this function, and use it and
struct ns_rr_cursor in gaih_getanswer_slice.  Adjust gaih_getanswer
and gaih_getanswer_noaaaa accordingly.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>