dhcp-server: implement captive portal option (RFC8910)

Clients observing the URI value "urn:ietf:params:capport:unrestricted"
may forego time-consuming forms of captive portal detection.

https://datatracker.ietf.org/doc/html/rfc8910

dhcp-server/dhcpd.conf

@@ -8,6 +8,7 @@ option space ubnt;
 option space omada;
 option ubnt.unifi-address code 1 = ip-address;
 option omada-controller-address code 138 = ip-address;
+option captive-portal-url code 114 = text;
 
 class "ubnt" {
         match if substring (option vendor-class-identifier, 0, 4) = "ubnt";
@@ -44,6 +45,7 @@ shared-network "{{ name }}" {
                         option omada-controller-address 5.1.66.255;
                 }
 		option interface-mtu            1280;
+		option captive-portal-url "urn:ietf:params:capport:unrestricted";
 		{%- endif  %}
 		{%- endfor  %}
 	}