libdns: Avoid using compound literals.

* dirmngr/dns.c (dns_inet_pton, dns_so_tcp_keep): Use automatic
variables.
(dns_poll, dns_send_nopipe): Likewise, adding const qualifier.

Compound literals is a feature of C99. Because we only use C90 plus
some limited features, in the project, it's better to avoid it.

Besides, we make sure when it's read-only.

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