MountInfo does not unescape special characters

See `unmangle_octal `function in PR #261

To test:

```
mkdir "$(echo -e 'aaa\\b\tc\nd#e')"
dd if=/dev/zero bs=1M count=200 of=ext4.img
mkfs.ext4 ext4.img
sudo mount ext4.img aaa*
```

Then run example `mountinfo`, this will give:
```
/dev/loop0 on /root/test/aaa\134b\011c\012d#e type ext4 (relatime,rw,seclabel)
```