btrbk cannot parse `/proc/self/mountinfo` if a mount contains space(s)

I guess technically it could also be the spaces in the path, but I do hope that’s not it ;)

```
ERROR: Failed to parse "/proc/self/mountinfo"
Offending line: 289 45 0:173 / /home/xxx/.local/share/Steam/steamapps/common/Baldurs\040Gate\0403 rw,relatime shared:232 - overlay overlayfs rw,seclabel,lowerdir=/home/xxx/.local/share/Steam/steamapps/compatdata/1086940/pfx/drive_c/users/steamuser/AppData/Local/Larian Studios/current/override:/home/xxx/.local/share/Steam/steamapps/common/bg3base,upperdir=/tmp/bg3/upper,workdir=/tmp/bg3/work
```

Start by locating the code that parses /proc/self/mountinfo and reproduce the failure with the offending line shown in the issue, especially its escaped spaces and overlay paths. Trace how fields are split and verify that a mount containing spaces parses successfully; done means btrbk no longer reports a parse error for this input.

Written by the indexing model from the issue text.