When manifest is loaded from a file, newline separator is read and stored

When a manifest is loaded from a file, the newline separator as present in the file is stored in the manifest.
Before, `linesep` was a field of `ManifestSection`. This did not describe the reality well: sections of the same manifest should all have the same line ending. Now, `linesep` is a field of `Manifest`. This also simplified (or, better say, made possible) its correct processing.

A corresponding test added: reading/storing a manifest with DOS line endings.