Split off a new SOCKS5 connect() method from Dial.

connect() takes an existing connection to a socks5 server, and
commands the server to extend that connection to a given target
address and port.

Change-Id: I5dbba58a67a0d884bda3d3ac194dc18bdebe74ab
Reviewed-on: https://go-review.googlesource.com/36643Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>