* dirmngr/dirmngr.h (struct opt): Add field 'disable_ipv6'.
* dirmngr/dirmngr.c (oDisableIPv6): New const.
(opts): New option --disable-ipv6.
(parse_rereadable_options): Set that option.
* dirmngr/dns-stuff.c (opt_disable_ipv6): New var.
(set_dns_disable_ipv6): New.
(resolve_name_standard): Make use of it.
* dirmngr/ks-engine-finger.c (ks_finger_fetch): Take care of
OPT.DISABLE_IPV6.
* dirmngr/ks-engine-hkp.c (map_host): Ditto.
(send_request): Ditto.
* dirmngr/ks-engine-http.c (ks_http_fetch): Ditto.
* dirmngr/ocsp.c (do_ocsp_request): Ditto.