Redesign VFS around handles

kernel/proc/procgroup.h

@@ -33,6 +33,8 @@ struct procgroup {
   uintptr_t map_base;
   struct procgroup_tls tls;
   uint64_t capabilities;
+  struct rb_node_link* vfs_handle_tree;
+  int sys_vfs_handles;
 };
 
 struct procgroup* procgroup_create (void);