fuse/SUBDIR-NAME by doing

  echo ZIP-FILE-NAME > fuse/config/SUBDIR-NAME

this exercises the PathFileSystemConnector mount functionality.

* Lookup a directory before trying to mount it.

* Cleanup stream closing for PathFileSystemConnector.

This makes sure we never take ownership of read only data from a file
system.

* Always return synthetic inode numbers in Lookup/Getattr

* Always return FUSE_UNKNOWN_INO in Readdir

* Use (chan DirEntry) interface for Readdir in
  PathFileSystem.OpenDir

Extends raw API with Release() and ReleaseDir() methods.