This is necessary to work around apparent bugs in the OSX FUSE
implementation.

OSXFUSE requires STATFS to always be present.

This protects against deadlock should the kernel issue a lookup
synchronously in response. This behavior has been seen on Linux
3.15.3-200.fc20.x86_64.

CreateIn, MknodIn, SetXAttrIn, and GetXAttrIn are separated into Linux and
Darwin-specific versions.

Tested on OSX 10.9.3 with OSXFuse 2.6.4 and Go 1.2.1.

OpenIn is updated to match Linux and OSXFuse definitions - changing the Unused
field to Mode.

This protects against deadlocks if the kernel calls back into
userspace in the notify handler.

To update, change

   nodefs.MountFileSystem(dir, fs, opts)

to

   nodefs.MountRoot(dir, fs.Root(), opts)

This makes it impossible to forget to create an Inode to match the
FsNode.

This is an API change. To update, update function signatures, and change
  
  me.Inode().NewChild("name", isdir, child)
  return child, fuse.OK

to

  return me.Inode().NewChild("name", isdir, child), fuse.OK

in Mkdir, Mknod, Create, Symlink and Link methods.

This obviates the nodefs.FileSystem type. To adjust for this API change,

  * call nodefs.MountRoot() on the Root() node of a to-be-mounted
    filesystem.

  * Move OnMount/OnUnmount methods to the type of the root node.

  * Stop embedding NewDefaultFileSystem() into file system
    objects. Usually FS objects no longer have to be exposed.