Revised slack-desc check, and four additional checks

Here's a revised slack-desc_check.sh as discussed.

There are four more new checks too:
install-dir_check.sh just checks that install/ exists
ownership_check.sh checks that everything in certain dirs is root:root (probably needs tuning)
symlink_check.sh checks there are no symlinks
tar113_check.sh checks that the package is compatible with installpkg

Would it be better to make install-dir and tar113 into a single valid-package check?