cache_options puts checksum files into etc, instead of var

A change was introduced in commit 04cd3f92 that puts the checksum cache into /etc/chef/cache. This is surprising to Unix/Linux users who expect cached files to be in /var/chef/cache or /var/cache/chef (depending on their adhearance to FHS and the like). The default location should be /var/chef/cache.

To work around this issue, in /etc/chef/client.rb or /etc/chef/solo.rb:

cache_options({ :path => "/var/chef/cache/checksums" })

The proper location for caching files on Windows is less clear, but we should make the default match since Chef::Config will use platform_specific_path().