lib/repo: Add locking auto cleanup handler

Define an auto cleanup handler for use with repo locking. This is based
on the existing auto transaction cleanup. A wrapper for
ostree_repo_lock_push() is added with it. The intended usage is like so:

  g_autoptr(OstreeRepoAutoLock) lock = NULL;
  lock = ostree_repo_auto_lock_push (repo, lock_type, cancellable, error);
  if (!lock)
    return FALSE;

The functions and type are marked to be skipped by introspection since I
can't see them being usable from bindings.

Closes: #1343
Approved by: cgwalters

src/libostree/libostree-experimental.sym

@@ -94,6 +94,8 @@ LIBOSTREE_2017.14_EXPERIMENTAL {
 global:
   ostree_remote_get_type;
   ostree_remote_get_url;
+  ostree_repo_auto_lock_cleanup;
+  ostree_repo_auto_lock_push;
   ostree_repo_lock_pop;
   ostree_repo_lock_push;
 } LIBOSTREE_2017.13_EXPERIMENTAL;