Clarify opkg's messages related to downloads:
* more visible error message for package list download failure
* separate error message for signature file download error
* if wget returns 4, signal the network error more clearly
* remove '.' from end of filenames and URLs
* try signature check only if the package list was downloaded ok.
Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
