fileio: use take_fdopen_unlocked()

This fixes maybe-uninitialized warning:
```
../src/basic/fileio.c: In function ‘chase_symlinks_and_fopen_unlocked’:
../src/basic/fileio.c:1026:19: warning: ‘f’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 1026 |         *ret_file = f;
      |         ~~~~~~~~~~^~~

```