This CL corresponds to golang.org/cl/118096 (7fbc8df48a7)
in the vgo repo.

It copies the bulk of the code from vgo back into the main repo,
but completely disabled - vgo.Init is a no-op and vgo.Enabled
returns false unconditionally.

The point of this CL is to make the two trees easier to diff and
to make future syncs smaller.

Change-Id: Ic34fd5ddd8272a70c5a3b3437b5169e967d0ed03
Reviewed-on: https://go-review.googlesource.com/118095Reviewed-by: Bryan C. Mills <bcmills@google.com>