dirmngr: New option --disable-ipv6

dirmngr: New option --disable-ipv6

* 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.

Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit 3533b854408fa93734742b2ee12b62aa0d55ff28)

Gbp-Pq: Name 0063-dirmngr-New-option-disable-ipv6.patch