libdns: Let kernel to decide the local port.

* dirmngr/dns.c (LEAVE_SELECTION_OF_PORT_TO_KERNEL): New.
(dns_socket): Don't select ephemeral port in user space.

Cherry picked from master commit:

861f1da0731bf29dcb9221c4f22c76b40ec15a78

There is no good reason to bind local port aggressively. It might be
some reason to do so, then, a user can specify it in /etc/resolv.conf
by the second argument of "interface" directive.

At least, it causes a problem on Windows. Binding a specified port in
user space can trigger the Firewall dialog on Windows. Since it can
be considered valid question, it is better not to bind with an
ephemeral port which is selected in user space, by default.

• GnuPG-bug-id: T3610
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>