ssh: drain stderr on forwarded TCP and Unix channels

tcpListener.Accept, unixListener.Accept, Client.dial and
Client.dialStreamLocal only read the main stream of the channels they
return. Data sent by the peer on the extended (stderr) stream
accumulates in the extPending buffer and its window credit is only
returned by ReadExtended, which is never called. Since the window is
shared between the two streams, a misbehaving peer can pin up to 2 MiB
per channel and eventually stall it; well-behaved peers never send
stderr on these channel types.

Drain the stderr stream into io.Discard, mirroring CL 783720 which
fixed the same issue in the agent forwarders.

The drain takes precedence over reading Stderr through a type assertion
on the returned net.Conn, which was never a supported use. For
comparison, OpenSSH refuses extended data on these channel types and
drops the payload without buffering it.

Updates golang/go#80333

Change-Id: I4f1445eee9ce1b56ca62cec342812d63cfdd3e13
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/802900
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>