[vnet] fix: close proxied channel only after data and requests are complete

[nklaassen]

This PR fixes two bugs in VNet's SSH proxy.

The first is that in the existing version utils.ProxyConn will close each proxied SSH channel as soon as it reaches EOF reading from either channel. First of all this is logically racy, if the source channel is closed the proxy may react to an EOF and close the target channel before the final channel request has been forwarded. It's also just incorrect, SSH clients or servers can send SSH_MSG_CHANNEL_EOF to signal that they are done sending data but they may still send channel requests that still need to be forwarded. This was causing simple commands like ssh user@server.cluster true to return a non-zero exit code because the channel was closed before the exit_status channel request was sent.

The fix for the first is to avoid closing the target channel until both the data and requests from the source channel have ended.

The second is that extended channel data of type SSH_EXTENDED_DATA_STDERR was not forwarded at all. This is necessary to forward stderr for SSH exec commands, it's the only extended data type defined in the spec or supported by golang.org/x/crypto/ssh. While changing the channel data forwarding logic here I thought it made sense to include the stderr data since it also effects the concurrency logic, otherwise the code you're reviewing here would be changing in a following PR.

I noticed both of these while trying Zed's remote SSH support via VNet SSH, on the bright side with these fixes in place it works fine.