manpage edits, mispellings and a feature.

Hi,

this pull request does three things:

1. It adds the missing --no-gtk-update-icon-cache switch in the manpage
2. It adds the install/slack-desc and install/doinst.sh files in the package database, if they are present in the package. That brings it in line with the original pkgtools and opens the possibility that someone can search for packages that have a doinst script for example (grep doinst.sh /var/log/packages/*)
3. It fixes several typos and mispellings.

Please review and pull if you like.