[tex-live] pretest -> update texlive 2017

[Karl Berry]

    Yes. But perhaps the relevant script could be made more
    robust by testing for a tex-archive path. 

Hi Bob - I don't feel that heuristic tinkering with the given path is
desirable, ie, guessing if something is a CTAN mirror url minus the
tlnet or not.

    The page titled Installing TeX Live over the Internet links
    to that page:

I added a few words about the need to append "systems/texlive/tlnet" to
the top-level mirror urls given on ctan.org/mirrors. hope it
suits. --thanks, karl.