merge Goswin Brederlow "support download of index files for different archs"

merge Goswin Brederlow "support download of index files for different archs"
patch which includes the following big changes:
- Declare the unused [vendor] field in sources.list as option field,
  e.g. deb [arch=amd64,i386 lang=en_GB have=fun] http://example.org
- When fetching index files download them for all APT::Architectures
  (overrideable with the options field above)
- Allow all architectures of APT::Architectures to be in the Cache
- Add the architecture to status and progress informations
- Add b= (Binary architecture) to policy

This commit doesn't incude the "pin-hack" as the Group structure will take
care of this (and does it already to some extend).