better subsystem support

[manfred-kaiser]

ssh supports subsystems. the most common is sftp, but others are also possible.

powershell as a subsystem:

https://docs.microsoft.com/en-us/powershell/scripting/learn/remoting/ssh-remoting-in-powershell-core?view=powershell-7

subsystem configuration:

https://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch05_07.htm

[manfred-kaiser]

When a client connects, a sftp channel will be opened. This can result in errors, when the server only supports one channel.
This issue is described in #45