Fixup minor things in list command.

Signed-off-by: Magnus Therning <magnus@therning.org>
Make list command take an optional package list.

With this change it's possible to limit the packages that are listed by the
list command.  Omitting package names results in the old behaviour, i.e. all
packages are listed.

Signed-off-by: Magnus Therning <magnus@therning.org>