[tlbuild] pruning platforms

[Karl Berry]

    ship xzdec for the platform nonetheless, and then:

Turns out that is already happening (and won't be changed), because of
the details of how we "prune" the platforms.

    - download the necessary binaries from CTAN mirror
    - take everything else from the DVD

In theory yes, but I can't fathom trying to implement such a thing for
such a rare case.  Certainly not now.

    platform and where he can find additional information about how to
    perform the installation if he wants to save bandwidth.

Not looking forward to writing those instructions.

k