consider deprecating "dialup" option

It is unclear if dialup statement is useful in practice, and at the same time it adds fair amount of logic to zone refresh/notify handling.

Consider the fun of finding out how following flags interact:

lib/dns/zone.c:

  19964 void
  19965 dns_zone_setdialup(dns_zone_t *zone, dns_dialuptype_t dialup) {
  19966         REQUIRE(DNS_ZONE_VALID(zone));
  19967 
  19968         LOCK_ZONE(zone);
  19969         DNS_ZONE_CLRFLAG(zone, DNS_ZONEFLG_DIALNOTIFY |
  19970                                        DNS_ZONEFLG_DIALREFRESH |
  19971                                        DNS_ZONEFLG_NOREFRESH);
  19972         switch (dialup) {
  19973         case dns_dialuptype_no:
  19974                 break;
  19975         case dns_dialuptype_yes:
  19976                 DNS_ZONE_SETFLAG(zone, (DNS_ZONEFLG_DIALNOTIFY |
  19977                                         DNS_ZONEFLG_DIALREFRESH |
  19978                                         DNS_ZONEFLG_NOREFRESH));
  19979                 break;
  19980         case dns_dialuptype_notify:
  19981                 DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_DIALNOTIFY);
  19982                 break;
  19983         case dns_dialuptype_notifypassive:
  19984                 DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_DIALNOTIFY);
  19985                 DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_NOREFRESH);
  19986                 break;
  19987         case dns_dialuptype_refresh:
  19988                 DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_DIALREFRESH);
  19989                 DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_NOREFRESH);
  19990                 break;
  19991         case dns_dialuptype_passive:
  19992                 DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_NOREFRESH);
  19993                 break;
  19994         default:
  19995                 UNREACHABLE();
  19996         }
  19997         UNLOCK_ZONE(zone);
  19998 }