pull: Add OSTREE_REPO_PULL_FLAGS_UNTRUSTED flag

If this is set we verify all objects we pull, even for local remotes,
and we avoid hard-linking into local source repos.

https://bugzilla.gnome.org/show_bug.cgi?id=764125

Closes: #221
Approved by: cgwalters

src/libostree/libostree.sym

@@ -328,8 +328,7 @@ global:
  *                         NOTE NOTE NOTE
  */
 
-/* UNCOMMENT WITH NEW SYMBOLS HERE:
 LIBOSTREE_2016.5 {
 global:
+        ostree_repo_import_object_from_with_trust;
 } LIBOSTREE_2016.4;
-*/