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.c

@@ -322,3 +322,20 @@ ostree_deployment_get_unlocked (OstreeDeployment *self)
 {
   return self->unlocked;
 }
+
+/**
+ * ostree_deployment_is_pinned:
+ * @self: Deployment
+ *
+ * See ostree_sysroot_deployment_set_pinned().
+ *
+ * Returns: `TRUE` if deployment will not be subject to GC
+ * Since: 2018.3
+ */
+gboolean
+ostree_deployment_is_pinned (OstreeDeployment *self)
+{
+  if (!self->origin)
+    return FALSE;
+  return g_key_file_get_boolean (self->origin, OSTREE_ORIGIN_TRANSIENT_GROUP, "pinned", NULL);
+}