Partial write in key-pinning-journal results in corrupted line
Tor sees a corrupted line in its `key-pinning-journal` because of a truncated line:
```
@opened-at 2016-08-07 20:49:24
wyD2E2ZG/fDQFbiQbz63VcvSKFo TNh6rQcairXqej0dOoRWOF93Zra+o+x+9b0VbiAG8zI
DahGjy7upvyovkp1sJ1C+/wKmT4 TNh6rQcairXqej0dOoRWOF93Zra+o+x+9b0VbiAG8zI
...
wyD2E2ZG/fDQFbiQbz63VcvSKFo TNh6rQcairXqej0dOoRWOF93Zra+o+x+9b0VbiAG8zI
DahGjy7upvyovkp1sJ1C+/wKmT4 TNh6rQcairXq
@opened-at 2016-10-05 20:02:15
DahGjy7upvyovkp1sJ1C+/wKmT4 TNh6rQcairXqej0dOoRWOF93Zra+o+x+9b0VbiAG8zI
```
Nick says Tor uses `fwrite()` when it should be using `write()` to write to that file.
issue