nixos/switch-to-configuration: Allow not filtering units

- Inspect what changed during these actions and print units that failed and

  that were newly started

By default, some units are filtered from the outputs to make it less spammy.

This can be disabled for development or testing by setting the environment variable

`STC_DISPLAY_ALL_UNITS=1`

Most of these actions are either self-explaining but some of them have to do

with our units or the activation script. For this reason, these topics are

explained in the next sections.

                    $units_to_start{$unit} = 1;

                    record_unit($start_list_file, $unit);

                    # Don't spam the user with target units that always get started.

                    if (($ENV{"STC_DISPLAY_ALL_UNITS"} // "") ne "1") {

                        $units_to_filter{$unit} = 1;

                    }

                }

            }

            # Stop targets that have X-StopOnReconfiguration set.

            # This is necessary to respect dependency orderings