go.net/proxy: fix SOCSK5 user/pass auth.
go.net/proxy: fix SOCSK5 user/pass auth.

The code was using the SOCKS version number (5) where it should have been using the username/password protocol version number (1).

With this change, username/password authentication actually works.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/10134044

fernandez14/go.net,andrewzeneski/go.net,go-mirrors/go.net,polydawn-ports/go.net,gigq/go.net,ssor/go.net