Add support for bind-mounting directories

* Add support function vfs_get_mount_point(), so a file system can get
  its own mount point (i.e. the node it covers). Re-added
  fs_mount::covers_vnode for that purpose -- the root node isn't know to
  the VFS before the mount() hook returns.
* Add function vfs_bind_mount_directory() which bind-mounts a directory
  to another. The Vnode::covers/covered_by mechanism is used, so this
  isn't true bind-mounting, but sufficient for what we need ATM and
  cheaper as well. The vnodes connected thus aren't tracked yet, which
  is needed for undoing the connection when unmounting.
* get_vnode_name(): Don't use dir_read() to read the directory. Since we
  have already resolved vnode to the covered vnode, we don't want the
  dirents to be "fixed" to refer to the covering nodes. Such a vnode
  simply wouldn't be found.

headers/private/kernel/vfs.h

@@ -146,6 +146,12 @@ status_t	vfs_create_special_node(const char *path, fs_vnode *subVnode,
 status_t	vfs_resolve_vnode_to_covering_vnode(dev_t mountID, ino_t nodeID,
 				dev_t *resolvedMountID, ino_t *resolvedNodeID);
 
+/* service calls for private file systems */
+status_t	vfs_get_mount_point(dev_t mountID, dev_t* _mountPointMountID,
+				ino_t* _mountPointNodeID);
+status_t	vfs_bind_mount_directory(dev_t mountID, ino_t nodeID,
+				dev_t coveredMountID, ino_t coveredNodeID);
+
 /* calls the syscall dispatcher should use for user file I/O */
 dev_t		_user_mount(const char *path, const char *device,
 				const char *fs_name, uint32 flags, const char *args,