sysupdate: Report download progress via sd_notify

We set up a NOTIFY_SOCKET to get download progress notifications from
each individual import helper. Along with the number of import jobs we
have to run, this gives an overall progress value which we report using
sd_notify

src/sysupdate/sysupdate.h

@@ -4,6 +4,9 @@
 #include <inttypes.h>
 #include <stdbool.h>
 
+/* Forward declare this type so that Transfers can point at it */
+typedef struct Context Context;
+
 extern bool arg_sync;
 extern uint64_t arg_instances_max;
 extern char *arg_root;