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/nss-systemd/userdb-glue.c

@@ -303,7 +303,7 @@ enum nss_status userdb_getgrnam(
         }
 
         if (!g) {
-                _cleanup_(_nss_systemd_unblockp) bool blocked = false;
+                _unused_ _cleanup_(_nss_systemd_unblockp) bool blocked = false;
 
                 if (strv_isempty(members))
                         return NSS_STATUS_NOTFOUND;
@@ -365,7 +365,7 @@ enum nss_status userdb_getgrgid(
         }
 
         if (!g) {
-                _cleanup_(_nss_systemd_unblockp) bool blocked = false;
+                _unused_ _cleanup_(_nss_systemd_unblockp) bool blocked = false;
 
                 /* So, quite possibly we have to extend an existing group record with additional members. But
                  * to do this we need to know the group name first. The group didn't exist via non-NSS