[tex-live] restarting TL installation

[Karl Berry]

To anyone interested -- please try the TL install again without
specifying -url.

Norbert changed the TL downloading to (a) fix on one mirror, and (b)
include a restart feature.  The general idea is that the downloaded
files are saved and if already present (and not corrupt), not downloaded
again.

At present, for the sake of testing they are never deleted, though we
will change that soon to delete everything on success.