Add DAFS docs to the Unix Quick Start guide

Add DAFS docs to the Unix Quick Start guide

Modify the sections dealing with creating the 'fs' bnode to also
mention how to create the 'dafs' bnode, and add an appendix explaining
DAFS and the differences between DAFS and non-DAFS.

Change-Id: I026405dbc3838a9a0247b85cbdc508d6be93275d
Reviewed-on: http://gerrit.openafs.org/2508
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>