Bug #172812 - Automatically uninstall packages to avoid blocker conflicts.

Bug #172812 - Automatically uninstall packages to avoid blocker conflicts.
The intention is to only uninstall packages that would be removed by
--depclean (after the updates) anyway. To prevent automatic uninstallation
of packages that the user probably wants to keep, the following types of
packages will not be automatically unmerged:

* Installed packages that have been pulled into the current dependency graph.
  Note that this check is not necessarily complete unless the --complete-graph
  option is enabled, making other sanity checks necessary.

* Installed packages matched by atoms from the system set. For safety,
  these are unmerged after being replaced by another version.

* Installed packages matched by atoms from the world set, if there is no
  matching replacement in the current dependency graph.

svn path=/main/trunk/; revision=9946