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/libostree-devel.sym

@@ -20,6 +20,8 @@
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 LIBOSTREE_2018.3 {
   ostree_deployment_origin_remove_transient_state;
+  ostree_sysroot_deployment_set_pinned;
+  ostree_deployment_is_pinned;
 } LIBOSTREE_2018.2;
 
 /* Stub section for the stable release *after* this development one; don't