Add slack-desc_check.sh.

This adds a check for slack-desc files -- hope it's ok for you.

Note that it currently accepts descriptions with between 3 and 13 lines, not the traditional 11 (installpkg accepts 13 or less lines, ever since Slackware 8.1) but you are welcome to change this in the obvious place. 

You might want to remove the "excessive blank lines" test; it's probably a bit too obsessive.