Properly show how much space is going to be free/required when deinstalling (which is the opposite of the installation case).

Testing one of my ports with the beta announced today, I noticed the following:

"Deinstallation has been requested for the following 1 packages:

        wine-gecko-2.21

The deinstallation will require 20 MB more space
[1/1] Deleting wine-gecko-2.21... done"

This is clearly incorrect, and looking at the code in pkg/utils.c I realized that
installation and deinstallation are handled the same when, really, they are the
respective opposites.