Add OS and arch to HTTP user-agent string

This makes the HTTP user-agent field look like so:

    User-Agent: cabal-install/1.21.0.0 (linux; x86_64)

This allows us to gather more statistics about our user-base as
with the current user-agent string we can only do a rough break-down by
cabal-install version numbers.
Merge pull request #1834 from hvr/user-agent-platform

Add OS and arch to HTTP user-agent string