systemctl: make sure legacy "reboot", "suspend" and friends are always asynchronous (#8848)

Currently, "reboot" behaves differently in setups with and without logind.
If logind is used (which is probably the more common case) the operation
is asynchronous, we should behave in the same way as "systemctl <verb>".
Let's clean this up, and always expose the same behaviour, regardless if
logind is used or not: let's always make it asynchronous.

See: #6479
Fixes: commit 130246d2e8

src/systemctl/systemctl.c

@@ -8466,6 +8466,11 @@ static int halt_main(void) {
                 }
         }
 
+        /* In order to minimize the difference between operation with and
+         * without logind, we explicitly enable non-blocking mode for this,
+         * as logind's shutdown operations are always non-blocking. */
+        arg_no_block = true;
+
         if (!arg_dry_run && !arg_force)
                 return start_with_fallback();