dirmngr: Fix Tor access for v6 addresses.

* dirmngr/http.c (use_socks): New.
(my_sock_new_for_addr): New.
(connect_server): Replace assuan_sock_new by my_sock_new_for_addr.

Libassuan always uses 127.0.0.1 to connect to the local Tor proxy.
https.c used to create a socket for the actual address family and thus
the connect call in Libassuan fails when it tries to connect to a v6
address using a v4 socket.

It would be cleaner to have the my_sock_new_for_addr function as a
public interface in Libassuan; for now we need to duplicate some code.
from Libassuan.

• GnuPG-bug-id: T2902
• Signed-off-by: Werner Koch <wk@gnupg.org>