libdns: For SOCKS connection, just fails.

* dirmngr/dns.c (dns_res_exec): If it's DNS_SO_SOCKS_CONN, don't
iterate to other server, but return the error immediately.

In the function libdns_switch_port_p in dns-stuff.c, this patch
allows to fallback using TOR_PORT2 correctly.

• Fixes-commit: bcdbf8b8ebe9d61160e0b007dabe1b6462ffbc93
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>