import,sysupdate: make notify event processed before SIGCHLD of worker processes

This fixes the following warning:
```
[    5.628796] TEST-13-NSPAWN.sh[299]: + importctl import-raw --class=confext /var/tmp/importtest
(snip)
[    5.638894] systemd-importd[302]: (transfer1) Operation completed successfully.
[    5.640760] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^MTotal:  40%
[    5.638902] systemd-importd[302]: (transfer1) Exiting.
[    5.638931] systemd-importd[302]: Got percentage from client: 40%
[    5.638956] systemd-importd[302]: Transfer process succeeded.
[    5.638988] systemd-importd[302]: Got notification datagram from unexpected peer, ignoring.
```

src/import/importd.c

@@ -721,7 +721,7 @@ static int manager_new(Manager **ret) {
 
         r = notify_socket_prepare(
                         m->event,
-                        SD_EVENT_PRIORITY_NORMAL,
+                        SD_EVENT_PRIORITY_NORMAL - 1, /* Make this processed before SIGCHLD. */
                         manager_on_notify,
                         m,
                         &m->notify_socket_path);

src/sysupdate/sysupdated.c

@@ -1746,7 +1746,7 @@ static int manager_new(Manager **ret) {
 
         r = notify_socket_prepare(
                         m->event,
-                        SD_EVENT_PRIORITY_NORMAL,
+                        SD_EVENT_PRIORITY_NORMAL - 1, /* Make this processed before SIGCHLD. */
                         manager_on_notify,
                         m,
                         &m->notify_socket_path);