network: Introduce UseCaptivePortal DHCPv4 option

Accepts a boolean. When enabled, UseCaptivePortal will request and
retain the captive portal configuration from the DHCP server.

src/libsystemd/sd-network/sd-network.c

@@ -258,6 +258,10 @@ int sd_network_link_get_sip(int ifindex, char ***ret) {
         return network_link_get_strv(ifindex, "SIP", ret);
 }
 
+int sd_network_link_get_captive_portal(int ifindex, char **ret) {
+        return network_link_get_string(ifindex, "CAPTIVE_PORTAL", ret);
+}
+
 int sd_network_link_get_search_domains(int ifindex, char ***ret) {
         return network_link_get_strv(ifindex, "DOMAINS", ret);
 }