lib: Add ostree_sysroot_load_if_changed() API

This will allow daemons like rpm-ostree to detect if there are any new
deployments efficiently, in combination with using inotify.  If there
are any changes, rpm-ostree wants publish them on DBus.

While we're here, add some changes to start doing unit C testing of
the sysroot API.

src/libostree/libostree.sym

@@ -317,4 +317,5 @@ global:
         ostree_repo_get_dfd;
         ostree_repo_list_refs_ext;
         ostree_sysroot_init_osname;
+        ostree_sysroot_load_if_changed;
 } LIBOSTREE_2016.3;