manager.c: Restrict ListCategories to the configuration directory.

manager.c: Restrict ListCategories to the configuration directory.

When using the ListCategories AMI action, it was possible to traverse
upwards through the directories to files outside of the configured
configuration directory. This action is now restricted to the configured
directory and an error will now be returned if the specified file is
outside of this limitation.

Resolves: #GHSA-33x6-fj46-6rfh

UserNote: The ListCategories AMI action now restricts files to the
configured configuration directory.