sysroot: Clean up API

Now that we have a real GObject for the sysroot, we have a convenient
place to keep track of 4 pieces of state:

* The current deployment list
* The current bootversion
* The current subbootversion
* The current booted deployment (if any)

Avoid requiring callers to pass all of this around and load it
piecemeal; instead the new thing is ostree_sysroot_load().

src/libostree/ostree-sysroot-private.h

@@ -30,6 +30,13 @@ struct OstreeSysroot {
 
   GFile *path;
   int sysroot_fd;
+
+  gboolean loaded;
+  
+  GPtrArray *deployments;
+  int bootversion;
+  int subbootversion;
+  OstreeDeployment *booted_deployment;
 };
 
 gboolean
@@ -39,6 +46,13 @@ _ostree_sysroot_read_boot_loader_configs (OstreeSysroot *self,
                                           GCancellable  *cancellable,
                                           GError       **error);
 
+gboolean
+_ostree_sysroot_read_current_subbootversion (OstreeSysroot *self,
+                                             int            bootversion,
+                                             int           *out_subbootversion,
+                                             GCancellable  *cancellable,
+                                             GError       **error);
+
 gboolean
 _ostree_sysroot_parse_deploy_path_name (const char *name,
                                         char      **out_csum,