[tex-live] Bugs in tlmgr-gui.c

[Karl Berry]

    Perhaps release-texlive.txt does not (yet) exist because it's still
    pretest.

It should exist, in the pretest and everywhere else.  It's in the
installer package (00texlive.installer.tlpsrc).  However, I see we never
put it into texlive.infra or any other normal package.  I added it to
texlive.infra.  We'll see if that breaks the world.

In any case, of course it is good to be robust against its nonexistence
and get the version info from texlive.tlpdb.  -k