sysroot: Add API to clean up transient keys in origin files

The `origin/unlocked` and `origin/override-commit` keys are examples of state
that's really transient; we don't want to maintain them across upgrades. Right
now there are bits for this in both `ostree admin upgrade` as well as in
rpm-ostree.

This new API will slightly clean up both cases, but it's really prep for adding
a concept of deployment "pinning" that will live in the new
`libostree-transient` group.

Closes: #1464
Approved by: jlebon

src/libostree/libostree-devel.sym

@@ -19,6 +19,7 @@
 
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 LIBOSTREE_2018.3 {
+  ostree_deployment_origin_remove_transient_state;
 } LIBOSTREE_2018.2;
 
 /* Stub section for the stable release *after* this development one; don't