Fix how 'Lof()' behaves on special files, and how read watches are automatically disabled or not. Add a new "Unknown stream size" error.

[INTERPRETER]
* NEW: New "Unknown stream size" error when 'Lof()' cannot know the size of a stream.
* BUG: If we can't know the size of a stream watched for reading, then we don't unwatch it automatically anymore.
* BUG: Streams based on non-regular file descriptors (devices for example) opened in direct mode now never use 'fstat()' for getting the file length.