http: auto-configure for local Tor proxy if called as 'tor'

With apts http transport supporting socks5h proxies and all the work
in terms of configuration of methods based on the name it is called with
it becomes surprisingly easy to implement Tor support equally (and
perhaps even a bit exceeding) what is available currently in
apt-transport-tor.

How this will turn out to be handled packaging wise we will see in
https://lists.debian.org/deity/2016/08/msg00012.html , but until this is
resolved we can add the needed support without actively enabling it for
now, so that this can be tested better.