graphdriver/unionfs: Add unionfs support to docker

This new graphdriver supports unionfs
(http://www.filesystems.org/project-unionfs.html) and the source code is
available at http://git.fsl.cs.sunysb.edu/. The patches are kept up to
date with the latest kernel releases so you can choose a particular
vanilla version. Otherwise, the patches are independent enough that they
can be applied on top of any distribution kernels and re-compiled.

Unionfs is very similar to AuFS and, in fact, is the base on which AuFS
was built. It doesn't support up to 128 layers like AuFS, being limited
to 42. Apart from that, it functions very closely to AuFS. It being a
simpler unioning filesystem to evaluate in production.

Hence, this unionfs graphdriver is based on AuFS graphdriver by making
slight modifications to account for differences between the two
filesystems.

Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@google.com>