* dirmngr/dirmngr.c (oDisableIPv4): New const.
(opts): New option --disable-ipv4.
(parse_rereadable_options): Set that option.
* dirmngr/dirmngr.h (opt): New field 'disable_ipv4'.
* dirmngr/dns-stuff.c (opt_disable_ipv4): bew var.
(set_dns_disable_ipv4): New.
(resolve_name_standard): Skip v4 addresses when OPT_DISABLE_IPV4 is
set.
* dirmngr/ks-engine-hkp.c (map_host): Ditto.
(send_request): Pass HTTP_FLAG_IGNORE_IPv4 if opt.disable_v4 is set.
* dirmngr/crlfetch.c (crl_fetch): Ditto.
* dirmngr/ks-engine-finger.c (ks_finger_fetch): Ditto.
* dirmngr/ks-engine-http.c (ks_http_fetch): Ditto.
* dirmngr/ocsp.c (do_ocsp_request): Ditto.