Eliminate erroneous warning when unstowing (#65)

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

NEWS

@@ -2,6 +2,15 @@ News file for Stow.
 
 * Changes in version 2.3.2
 
+*** Eliminated a spurious warning on unstowing
+
+    2.3.1 introduced a benign but annoying warning when unstowing
+    in certain circumstances.  It looked like:
+
+      BUG in find_stowed_path? Absolute/relative mismatch between Stow dir X and path Y
+
+    This was caused by erroneous logic, and has now been fixed.
+
 *** Improved debug output
 
     Extra output resulting from use of the -v / --verbose flag