systemctl: start_unit() returns > 0 on error, let's catch that properly

Let's make sure we fall back correctly to initctl when start_unit()
returns an error > 0.

Fixes: #11546

src/systemctl/systemctl.c

@@ -8939,7 +8939,7 @@ static int reload_with_fallback(void) {
 
 static int start_with_fallback(void) {
         /* First, try systemd via D-Bus. */
-        if (start_unit(0, NULL, NULL) >= 0)
+        if (start_unit(0, NULL, NULL) == 0)
                 return 0;
 
         /* Nothing else worked, so let's try /dev/initctl */