dhcp/internal: handle localhost and 0.0.0.0 DNS/NTP servers specially

- regarding the DHCP options, we should not suppress them. If the lease
  contains such bogus(?) addresses, we still want to expose them on
  D-Bus without modification.

- regrading using the DNS server, ignore localhost addresses like done for
  systemd-networkd ([1], [2]).

Until recently, the DHCP library would internally suppress such
addresses ([3]). That is no longer the case, and we should handle
them specially.

[1] Systemd-pull-requst #4524
[2] systemd/systemd@d9ec2e6
[3] Systemd-pull-request #11208

src/dhcp/nm-dhcp-systemd.c

@@ -326,17 +326,19 @@ lease_to_ip4_config (NMDedupMultiIndex *multi_idx,
 	if (num > 0) {
 		nm_gstring_prepare (&str);
 		for (i = 0; i < num; i++) {
-			if (addr_list[i].s_addr == 0)
-				continue;
-
 			nm_utils_inet4_ntop (addr_list[i].s_addr, addr_str);
 			g_string_append (nm_gstring_add_space_delimiter (str), addr_str);
+
+			if (   addr_list[i].s_addr == 0
+			    || nm_ip4_addr_is_localhost (addr_list[i].s_addr)) {
+				/* Skip localhost addresses, like also networkd does.
+				 * See https://github.com/systemd/systemd/issues/4524. */
+				continue;
+			}
 			nm_ip4_config_add_nameserver (ip4_config, addr_list[i].s_addr);
 		}
-		if (str->len) {
-			LOG_LEASE (LOGD_DHCP4, "nameserver '%s'", str->str);
-			add_option (options, dhcp4_requests, SD_DHCP_OPTION_DOMAIN_NAME_SERVER, str->str);
-		}
+		LOG_LEASE (LOGD_DHCP4, "nameserver '%s'", str->str);
+		add_option (options, dhcp4_requests, SD_DHCP_OPTION_DOMAIN_NAME_SERVER, str->str);
 	}
 
 	num = sd_dhcp_lease_get_search_domains (lease, (char ***) &search_domains);