Add 'VCS' key

Add 'VCS' key

Spec files have a lot of metadata about a project.  However one of the
most key components is the upstream version control system which was
notably lacking.

Resolve this by adding a "VCS" key.  There is no specification
for contents of this key, given that the set of version control
systems (and features thereof) are not well-defined.  However,
recommendations are:

 * git: This URL should be in a form that can be passed to "git clone",
   with the additional feature that an optional fragment identifier "#foo"
   denotes a branch or tag.