fstab module is unsafe

The fstab module does this:

```
class Fstab(io.FileIO):
    # [...]
    def remove_entry(self, entry):
        # [...]
        self.seek(0)
        self.write(''.join(lines).encode('us-ascii'))
        self.truncate()
```

If a crash occurs at the right time, and the write was large enough to be split into multiple operations, or if the crash occurs after write and before truncate, /etc/fstab could have trailing garbage and may be sufficiently damaged so as to cause problems booting.

Instead the file should be always updated using the write-new-file/fsync/rename/fsync pattern.

(Depending on which filesystem developer you're talking to, the weather, and the condition of the Sun's corona, you may be recommended different recipes. But the current methodology is certainly flawed.)