basic: split out sync() family of calls from fs-util.[ch] into new c/h file

No changes in code, just some splitting out.

src/basic/sync-util.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+int fsync_directory_of_file(int fd);
+int fsync_full(int fd);
+
+int fsync_path_at(int at_fd, const char *path);
+int fsync_parent_at(int at_fd, const char *path);
+int fsync_path_and_parent_at(int at_fd, const char *path);
+
+int syncfs_path(int at_fd, const char *path);