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/ostree-sysroot-private.h

@@ -49,6 +49,7 @@ struct OstreeSysroot {
   int bootversion;
   int subbootversion;
   OstreeDeployment *booted_deployment;
+  struct timespec loaded_ts;
 
   /* Only access through ostree_sysroot_get_repo() */
   OstreeRepo *repo;