shared: add root argument to search_and_fopen

This adds the same root argument to search_and_fopen that
conf_files_list already has. Tools that use those two functions as a
pair can now be easily modified to load configuration files from an
alternate root filesystem tree.
tmpfiles: Add --root option to operate on an alternate fs tree.

This makes it possible to initialize or cleanup an arbitrary filesystem
hierarchy in the same way that it would be during system boot.
tmpfiles: Add --root to the man page.