networkd: serialize the private options

Save the private options along side the rest of the options in the lease
files.

src/libsystemd-network/network-internal.h

@@ -74,3 +74,6 @@ struct sd_dhcp_route;
 
 void serialize_dhcp_routes(FILE *f, const char *key, struct sd_dhcp_route *routes, size_t size);
 int deserialize_dhcp_routes(struct sd_dhcp_route **ret, size_t *ret_size, size_t *ret_allocated, const char *string);
+
+int serialize_dhcp_option(FILE *f, const char *key, const uint8_t *data, size_t size);
+int deserialize_dhcp_option(uint8_t **data, size_t *data_len, const char *string);