Add support for more selective pruning

There are use cases for having a single repo with branches
with different lifecycles; a simple example of what I was
trying to do in CentOS Atomic Host work is have "stable"
and "devel" branches, were we want to prune devel, but
retain *all* of stable.

This patch is split into two parts - first we add a low level "delete all
objects not in this set" API, and change the current prune API
to use this.

Next, we move more logic into the "ostree prune" command. This paves the way for
demonstrating how more sophisticated algorithms/logic could be developed outside
of the ostree core.

Also, the --keep-younger-than logic already lived in the commandline, so it
makes sense to keep extending it there.

Closes: https://github.com/ostreedev/ostree/issues/604

Closes: #646
Approved by: jlebon

apidoc/ostree-sections.txt

@@ -368,6 +368,7 @@ ostree_repo_commit_traverse_iter_next
 OstreeRepoPruneFlags
 ostree_repo_prune
 ostree_repo_prune_static_deltas
+ostree_repo_prune_from_reachable
 OstreeRepoPullFlags
 ostree_repo_pull
 ostree_repo_pull_one_dir