sysroot: Add concept of deployment "pinning" 📌

Example user story: Jane rebases her OS to a new major version N, and wants to
keep around N-1 even after a few upgrades for a while so she can easily roll
back. I plan to add `rpm-ostree rebase --pin` to opt-in to this for example.

Builds on the new `libostree-transient` group to store pinning state there.

Closes: https://github.com/ostreedev/ostree/issues/1460

Closes: #1464
Approved by: jlebon

src/libostree/ostree-deployment.h

@@ -74,6 +74,9 @@ _OSTREE_PUBLIC
 GKeyFile *ostree_deployment_get_origin (OstreeDeployment *self);
 
 
+_OSTREE_PUBLIC
+gboolean ostree_deployment_is_pinned (OstreeDeployment *self);
+
 _OSTREE_PUBLIC
 void ostree_deployment_set_index (OstreeDeployment *self, int index);
 _OSTREE_PUBLIC