systemd-notify: Always pass a valid pid to sd_pid_notify

If the option --pid was used, take the pid from this option, unless take
the parend pid. Using 0 as pid (ucred of systemd-notify) will result 99% of the
time in a failure with this error: "Cannot find unit for notify message of PID"

Shouldn't we use always the ppid, since the MAINPID is something else ?

Signed-off-by: Benjamin Robin <dev@benjarobin.fr>

src/notify/notify.c

@@ -191,7 +191,7 @@ int main(int argc, char* argv[]) {
                 goto finish;
         }
 
-        r = sd_pid_notify(arg_pid, false, n);
+        r = sd_pid_notify(arg_pid ? arg_pid : getppid(), false, n);
         if (r < 0) {
                 log_error_errno(r, "Failed to notify init system: %m");
                 goto finish;