Setup and cached data.

[Robert Collins]

I've disabled the cache-disabling code in setup in HEAD. This allows
even speedier retrieval of setup.ini|bz2 files when they have not
changed recently. I'm doing this on the theory that folk don't need
absolutely the latest-and-greatest ini and package files:

* Ini updates only occur when packages are released: this isn't daily,
and even if it was, the cache TTL logic would check automatically on
files so recent.
* Package files are versioned, any new file will always be a cache miss.

If anyone runs into trouble with this... Please let me know. It
shouldn't cause any issues though.

On a similar note, I plan on adding a 'current-date-stamp' check to
network retrieval, to allow hints for the current version setup has of a
file. I.e. if the last update of the file we downloaded was x, then only
bother with network retrieval for the fiel if it has been modified
since.

That is a longer term goal though... Patches welcome.

Rob