libglnx porting: Use glnx_shutil_rm_rf_at()

In some cases (such as `ostree-sysroot-cleanup.c`), the surrounding
code would be substantially cleaner if it was also ported to
fd-relative, but I'm going to do that in a separate patch.

That way these patches are easier to review for mechanical
correctness.  I used an Emacs keyboard macro as the poor man's
[Coccinelle](http://coccinelle.lip6.fr/).

src/libostree/ostree-repo-commit.c

@@ -1272,8 +1272,8 @@ rename_pending_loose_objects (OstreeRepo        *self,
         }
     }
 
-  if (!gs_shutil_rm_rf_at (self->tmp_dir_fd, self->commit_stagedir_name,
-                           cancellable, error))
+  if (!glnx_shutil_rm_rf_at (self->tmp_dir_fd, self->commit_stagedir_name,
+                             cancellable, error))
     goto out;
 
   ret = TRUE;
@@ -1323,7 +1323,7 @@ cleanup_tmpdir (OstreeRepo        *self,
       delta = curtime_secs - mtime;
       if (delta > 60*60*24)
         {
-          if (!gs_shutil_rm_rf (path, cancellable, error))
+          if (!glnx_shutil_rm_rf_at (AT_FDCWD, gs_file_get_path_cached (path), cancellable, error))
             goto out;
         }
     }