[pca] PCA return codes

[Martin Paul]

Martin Paul wrote:
>   * Check whether patch already exists before announcing download
> 
> If a patch already exists in patchdir, it will not say anything about 
> "Downloading .." anymore. Let me know if that's OK for you.

I discovered that this change had some ill side effects with parallel 
patch downloads (e.g. on a proxy or with multiple threads). I 
implemented it in a different way now, the main change to the original 
version is that pca writes "Looking for ..." instead of "Downloading 
..". This is closer to the truth anyway.

Martin.