Consistent handling of SSH_MSG_CHANNEL_REQUEST messages

Symptom:

SSH connection is kept open after the command completes when using Putty, Plink or TortoisePlink, or rather Putty does not finish.

See:

• https://code.google.com/p/tortoisegit/issues/detail?id=1880
• https://jira.atlassian.com/browse/STASH-3824

Cause:
ChannelSession has special handling for SSH_MSG_CHANNEL_REQUEST messages that are sent by Putty:

if (type != null && type.endsWith("@putty.projects.tartarus.org"))

{ // Ignore but accept, more doc at // http://tartarus.org/~simon/putty-snapshots/htmldoc/AppendixF.html return true; }

There are really two problems with this workaround that was introduced in SSHD-77:

1) This code ignores the "want reply" flag on the field and simply ignores the request. According to the SSH protocol specification, the server MUST send back a reply (success or failure) if the "want reply" flag is set to true.
2) Putty intentionally sends an invalid request and expects to receive a SSH_MSG_CHANNEL_FAILURE response (see http://tartarus.org/~simon/putty-snapshots/htmldoc/AppendixF.html), which is the default behaviour of ChannelSession.

I don't know whether changes in Putty or changes in MinaSSHD have made the workaround obsolete, but it is obsolete. I've verified that without the bit of code I quoted above, the Putty terminates normally.