tmpfile-util: introduce new CLEANUP_TMPFILE_AT() API

This should allow us to get rid of a bunch of "fail:" labels, because we
can clean up tmpfiles relative to some atfd this way.

This only ports over a small number of potential users, but there's more
work to be done.

src/basic/tmpfile-util.c

@@ -436,3 +436,17 @@ int mkdtemp_open(const char *template, int flags, char **ret) {
 
         return fd;
 }
+
+void cleanup_tmpfile_data_done(struct cleanup_tmpfile_data *d) {
+        assert(d);
+
+        if (!d->dir_fd ||
+            *d->dir_fd < 0 ||
+            !d->filename ||
+            !*d->filename)
+                return;
+
+        (void) unlinkat(*d->dir_fd, *d->filename, 0);
+        d->dir_fd = NULL;
+        d->filename = NULL;
+}