This change adds DialWithConn method for allowing package users to use
own net.Conn implementations optionally.

Also makes the deprecated Dialer.Dial return a raw transport connection
instead of a forward proxy connection for preserving the backward
compatibility on proxy.Dialer.Dial method.

Fixes golang/go#25104.

Change-Id: I4259cd10e299c1e36406545708e9f6888191705a
Reviewed-on: https://go-review.googlesource.com/110135
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>