* add `get_path_offset` to skip drive/protocol, serveur name and root dir.
* add `get_parent_offset` to get the parent of path component
* add `get_curdir` to hide OS dependencies and test `getcwd` failures
* add `get_homedir` for current or specified user
* simplify `make_user_path`, `canonicalize_absolute_buffer_path`,
`qe_set_user_option`