[tex-live] persistent connections when downloading

[Karl Berry]

    I saw an HTTP 500 error on one system this morning, so it fell back
    to wget.

Glad to know that the fallback worked :).

    What would be the best way to test for availability of this feature?  Tlmgr
    version >= 16424?  

Seems fine to me.

    Run `tlmgr --persistent-downloads list` and check for an error?

I suppose actually trying the option and making sure that it doesn't
output an error is protection against us distributing a broken
-persistent-downloads one day.  Your choice.


We'll probably push tlcritical to tlnet tomorrow for this change.  I
just want to review the doc strings.

Thanks,
Karl