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.h

@@ -51,6 +51,12 @@ gboolean ostree_sysroot_load (OstreeSysroot  *self,
                               GCancellable   *cancellable,
                               GError        **error);
 
+_OSTREE_PUBLIC
+gboolean ostree_sysroot_load_if_changed (OstreeSysroot  *self,
+                                         gboolean       *out_changed,
+                                         GCancellable   *cancellable,
+                                         GError        **error);
+
 _OSTREE_PUBLIC
 void ostree_sysroot_unload (OstreeSysroot  *self);