tree-wide: mark set-but-not-used variables as unused to make LLVM happy

LLVM 13 introduced `-Wunused-but-set-variable` diagnostic flag, which
trips over some intentionally set-but-not-used variables or variables
attached to cleanup handlers with side effects (`_cleanup_umask_`,
`_cleanup_(notify_on_cleanup)`, `_cleanup_(restore_sigsetp)`, etc.):

```
../src/basic/process-util.c:1257:46: error: variable 'saved_ssp' set but not used [-Werror,-Wunused-but-set-variable]
        _cleanup_(restore_sigsetp) sigset_t *saved_ssp = NULL;
                                                     ^
                                                     1 error generated.
```

src/core/manager.c

@@ -1758,7 +1758,7 @@ int manager_startup(Manager *m, FILE *serialization, FDSet *fds, const char *roo
 
         {
                 /* This block is (optionally) done with the reloading counter bumped */
-                _cleanup_(manager_reloading_stopp) Manager *reloading = NULL;
+                _unused_ _cleanup_(manager_reloading_stopp) Manager *reloading = NULL;
 
                 /* If we will deserialize make sure that during enumeration this is already known, so we increase the
                  * counter here already */
@@ -3275,7 +3275,7 @@ void manager_retry_runtime_watchdog(Manager *m) {
 }
 
 int manager_reload(Manager *m) {
-        _cleanup_(manager_reloading_stopp) Manager *reloading = NULL;
+        _unused_ _cleanup_(manager_reloading_stopp) Manager *reloading = NULL;
         _cleanup_fdset_free_ FDSet *fds = NULL;
         _cleanup_fclose_ FILE *f = NULL;
         int r;

src/core/namespace.c

@@ -853,7 +853,7 @@ static int mount_private_dev(MountEntry *m) {
         char temporary_mount[] = "/tmp/namespace-dev-XXXXXX";
         const char *d, *dev = NULL, *devpts = NULL, *devshm = NULL, *devhugepages = NULL, *devmqueue = NULL, *devlog = NULL, *devptmx = NULL;
         bool can_mknod = true;
-        _cleanup_umask_ mode_t u;
+        _unused_ _cleanup_umask_ mode_t u;
         int r;
 
         assert(m);