util: add new write_string_filef() helper

This new helper combines asprintf() and write_string_file() in one,
which is useful at various places to shorten the code a bit.

src/basic/fileio.c

@@ -206,6 +206,25 @@ fail:
         return 0;
 }
 
+int write_string_filef(
+                const char *fn,
+                WriteStringFileFlags flags,
+                const char *format, ...) {
+
+        _cleanup_free_ char *p = NULL;
+        va_list ap;
+        int r;
+
+        va_start(ap, format);
+        r = vasprintf(&p, format, ap);
+        va_end(ap);
+
+        if (r < 0)
+                return -ENOMEM;
+
+        return write_string_file(fn, p, flags);
+}
+
 int read_one_line_file(const char *fn, char **line) {
         _cleanup_fclose_ FILE *f = NULL;
         int r;