basic/fileio: extend atomic file writing with timestamp setting

There should be no functional change.

src/basic/fileio-label.h

@@ -24,7 +24,10 @@
 
 #include "fileio.h"
 
-int write_string_file_atomic_label(const char *fn, const char *line);
+int write_string_file_atomic_label_ts(const char *fn, const char *line, struct timespec *ts);
+static inline int write_string_file_atomic_label(const char *fn, const char *line) {
+        return write_string_file_atomic_label_ts(fn, line, NULL);
+}
 int write_env_file_label(const char *fname, char **l);
 int fopen_temporary_label(const char *target,
                           const char *path, FILE **f, char **temp_path);