Use description of installed package if it's not available anymore

Use description of installed package if it's not available anymore

* dselect/pkginfo.cc (packagelist::itd_description), dselect/pkgtop.cc
(packagelist::redraw1itemsel): Use description of installed
package as a fallback if the package is not more listed in the
available list.

Based on a patch by Bruce Sass <bmsass@shaw.ca>.