apt: add switch for download cache directory

[maphew]

Apt could use an optional parameter to specify the location of the package cache directory. This will make it easier to test concurrent o4w installs on the same machine, or different machines across a network, and lighten the bandwith load by letting them share the same cache directory.

Also consider this: if --cache=... is used with "apt setup" the cache location is written into .../etc/setup/lastcache, which would help #43