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.h

@@ -39,6 +39,8 @@ static inline int write_string_file(const char *fn, const char *line, WriteStrin
         return write_string_file_ts(fn, line, flags, NULL);
 }
 
+int write_string_filef(const char *fn, WriteStringFileFlags flags, const char *format, ...) _printf_(3, 4);
+
 int read_one_line_file(const char *fn, char **line);
 int read_full_file(const char *fn, char **contents, size_t *size);
 int read_full_stream(FILE *f, char **contents, size_t *size);