dirmngr: Use IPv4 or IPv6 interface only if available.

* dirmngr/dns-stuff.c (cached_inet_support): New variable.
(dns_stuff_housekeeping): New.
(check_inet_support): New.
* dirmngr/http.c (connect_server): Use only detected interfaces.
* dirmngr/dirmngr.c (housekeeping_thread): Flush the new cache.

This currently works only for Windows but that is where users really
ran into problems. The old workaround was to configure disable-ipv4
or disable-ipv6.

• Signed-off-by: Werner Koch <wk@gnupg.org>