Bug#4450: dselect package downgrades

[Barak Pearlmutter <bap@valaga.salk.edu>]

Package: dpkg
Version: 1.2.14elf

dselect knows about upgrades (packages on the currently available
medium that are upgrades of currently installed version) and offers
the user the option of taking advantage of them.  But it doesn't have
much notion of packages that are available on the current medium but
are *less* current than the currently installed versions.  dselect
should make a point of preventing users from inadvertently downgrading
packages, and (in order to ease their nerves) should, when
appropriate, make the user aware that this functionality is present.

The current behavior, with lines like

 *== Req base     mount        2.5l-1      2.5j-1      Tools for mounting and m

listed as simply among

    --- Up to date installed packages ---

is scary.  It should be under something like

    --- Super-current installed packages ---

so the user knows that dselect won't go off downgrading things without
serious explicit user overrides of default behavior being applied.