FsNode has no Inode.

If not (in case of hardlink), increase the lookupCount for the inode.

To use hard-links, the FileSystem should return consistent
FileInfo.Ino data from the GetAttr call.

* Always take R/W lock

* If the child exists, do addLookupCount() before GetAttr, so the
  child can't be cleaned up during the unprotected GetAttr() call.

errors.

* Introduce Inode.addLookupCount(), checking for underflow. 

* Increase lookupCount for mounts only once

* If FsNode.Link() returns existing node, increase its lookup count.

* Root node should have lookupCount = 1 at start.

* If we get a Lookup for an existing node, increase its lookup count.

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.