libdns: Avoid using compound literals (7).

* dirmngr/dns.h (DNS_OPTS_INIT, dns_opts): Remove.
* dirmngr/dns-stuff.c (libdns_res_open): Use zero-ed, and initialized
automatic variable for opts.
* dirmngr/dns.c (send_query, resolve_query, resolve_addrinfo):
Likewise.

In fact, DNS_OPTS_INIT was only needed when args are none. With
partially specified initialization, C99 guarantees zero-ed other
members just like static object.

(cherry picked from commit d661acd483236d34720a4959fc816d05f89c2cb7)

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>