Cache handling

When pm-enable calls pm-download, is passes the --cache option. This is fine, although unnecessary for the wget package handler, which seems to use the cache invariantly, regardless of the setting of the --cache option. The git package handler, on the other hand, creates a git reference cache if --cache is set.

It seems to me that the --cache option should be removed from em-enable's invoke of pm-download, as this could have unintended consequences (e.g. if $options['package-handler'] = 'git_drupalorg' is permanently set in drushrc.php).

Another issue is that drush interprets --cache as a global option for downloading files, and it also interprets --cache as an sql-sync option, containing the number of hours to hold a dump in the cache. These two definitions can be in conflict.

This conflict should be resolved somehow. The other question is, how should the --cache global option affect caching, and should internal this flag be propagated in backend invoke calls in the same way that --include is (i.e., only on the same machine)?