policy: get the DHCPv6 hostname from the FQDN option

There isn't any 'host-name' option for DHCPv6. Read instead the
'fqdn-fqdn' option that carries the FQDN assigned by the server to the
client.

(cherry picked from commit 1f74ea52)
(cherry picked from commit 4e1da002)

src/nm-policy.c

@@ -766,7 +766,7 @@ update_system_hostname (NMPolicy *self, const char *msg)
 		/* Grab a hostname out of the device's DHCP6 config */
 		dhcp_config = nm_device_get_dhcp_config (get_default_device (self, AF_INET6), AF_INET6);
 		if (dhcp_config) {
-			dhcp_hostname = nm_dhcp_config_get_option (dhcp_config, "host_name");
+			dhcp_hostname = nm_dhcp_config_get_option (dhcp_config, "fqdn_fqdn");
 			if (dhcp_hostname && dhcp_hostname[0]) {
 				p = nm_str_skip_leading_spaces (dhcp_hostname);
 				if (p[0]) {