When unstowing a package, cleanup_invalid_links() is invoked to remove
any invalid links owned by Stow. It was invoking link_owned_by_package()
to check whether each existing link is owned by Stow. This in turn
called find_stowed_path() which since 40a0807185 was not allowing for
the possibility that it could be passed a symlink *not* owned by Stow
with an absolute target and consequently emitting an erroneous warning.
So remove this erroneous warning, and refactor find_stowed_path()
to use two new helper functions for detecting stow directories:
link_dest_within_stow_dir() and find_containing_marked_stow_dir().
Also refactor the logic within each to be simpler and more accurate,
and add more test cases to the corresponding parts of the test suite.
Fixes#65.
Closes#103.
https://github.com/aspiers/stow/issues/65
