Add support for DHCP static route options

This adds support for DHCP options 33 and 121: Static Route and
Classless Static Route. To enable this feature, set UseRoutes=true
in .network file. Returned routes are added to the routing table.

src/network/networkd-network.c

@@ -89,6 +89,7 @@ static int network_load_one(Manager *manager, const char *filename) {
         network->dhcp_dns = true;
         network->dhcp_hostname = true;
         network->dhcp_domainname = true;
+        network->dhcp_routes = true;
 
         r = config_parse(NULL, filename, file, "Match\0Network\0Address\0Route\0DHCPv4\0", config_item_perf_lookup,
                         (void*) network_network_gperf_lookup, false, false, network);