[tlbuild] request for modest enhancement of tlmgr

[Karl Berry]

            ./install-tl: No binary platform specified/available,
            proceeding with fingers crossed

I'm sorry, but I just don't think it's a good idea to "proceed" without
known binaries. It doesn't seem like anything useful can happen.

            You may be able to find binaries for your system at 
            http://www.math.utah.edu/pub/texlive-utah/

Good idea. I changed the message to refer to
https://tug.org/texlive/custom-bin.html,
where there are some prominent links to Utah.

    Can tlmgr please be tweaked to support DragonFlyBSD?

Hopefully done for the next pretest build, which will start soon.
Let me know if problems. --thanks, karl.