sd-network: add sd_network_linkg_get_domains()

For now this only exposes the domain name (DHCP Option 15), and not
the search string (DHCP Option 119), which will be implemented in
a follow-up patch.

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

@@ -206,6 +206,10 @@ _public_ int sd_network_link_get_ntp(int ifindex, char ***ret) {
         return network_get_link_strv("NTP", ifindex, ret);
 }
 
+_public_ int sd_network_link_get_domains(int ifindex, char ***ret) {
+        return network_get_link_strv("DOMAINS", ifindex, ret);
+}
+
 static inline int MONITOR_TO_FD(sd_network_monitor *m) {
         return (int) (unsigned long) m - 1;
 }