opkg: encode archive filenames while constructing download URLs

opkg: encode archive filenames while constructing download URLs

Various web servers, namely Amazon S3 ones, have problems handling requests
to URLs with a literal "+" in the path component.

According to the RFC 3986 "+" is a reserved char and its purpose is
delimiting. When used in a file name it should be encoded.

Use the new urlencode_path() helper to encode the path component before
constructing the final download URL.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Tested-by: Rafał Miłecki <rafal@milecki.pl>