Add PathNodeFs.Mount/Unmount methods.

- Pass PathNodeFs in FileSystem.Mount()

- Add Inode.Files(mask)

- Reopen all files with layer > 0 on promotion.

* NodeFileSystem: interface for defining filesystems in terms of inodes.

* FsNode: interface for inodes to be used by NodeFileSystem

* PathNodeFs: NodeFileSystem that assumes bijective mapping between
  inodes and paths

* Change inode to Inode, a public (but mostly opaque) type.

Use inodeFs as layer between FileSystemConnector and FileSystem.

This further prepares fsInode as an intermediate layer between raw and
path filesystems.

Only fsInode maintains Parent links. This prepares for inserting
a layer between RawFileSystem and FileSystem, which can define
filesystems in terms of inodes.