This change adds code to report the reclaimed space after a prune.
Reclaimed space from volumes, images, and containers is recorded
during the prune call in a PruneReport struct. These structs are
collected into a slice during a system prune and processed afterwards
to calculate the total reclaimed space.
Closes#8658
Signed-off-by: Baron Lenardson <lenardson.baron@gmail.com>