* 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.

PathFileSystemConnector file system.

This reduces the overhead of allocating, zeroing and GC-ing large
amounts of memory.

For large reads into /dev/null, this provides a 22% speed-up.

This is a file system that will composite multiple FUSE
filesystems, so you can have for example

 /dir1   -> one simple FUSE filesystem
 /dir2   -> another simple FUSE filesystem
 /config -> a FS interface to daemon configuration

Each of these directories should be RawFileSystem instances.  This
class takes care of the mapping between global inode ids and inode ids
of each sub-FUSE filesystem.

No files or directories may be created in the toplevel dir. Instead,
the daemon should issue AddFileSystem() and RemoveFileSystem()
methods internally.  This could be done in response to writes in a
/config directory.