Placing a .stow file in a directory tells Stow that this directory
should be considered a Stow directory. This is already
well-documented.
There was an undocumented and slightly broken feature where placing a
.nonstow file in a directory was treated in exactly the same way. The
intention was for .nonstow to cause Stow to skip stowing into and
unstowing from that directory and any of its descendants. However, it
also caused Stow to consider symlinks into any of those directories as
owned by Stow, even though that was clearly not the intention. So
separate treatment of .stow and .nonstow markers, so that while both
provide protection against Stow stowing and unstowing, only .stow
affects the symlink ownership logic in find_stowed_path() and
marked_stow_dir().
Probably no one uses the undocumented .nonstow feature, so it may make
sense to remove this in future.
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