When a Channel of type tcpip-forward is closed, it does not emit the closure request

I'd suggest the channel to keep track of the channel being explicitly closed by the user or third party. When it _is_ explicitly closed, I'd send the "cancel-tcpip-forward" global request as described on [RFC 4254 page 17](https://ietf.org/rfc/rfc4254.txt). This way, the user does not need to concern itself with protocol specific knowledge. This allows libraries to wrap a channel without requiring awareness of the SSH protocol.

Start by locating the tcpip-forward channel close handling and compare its behavior with RFC 4254 page 17. Trace how explicit closure by the user or third party is represented, then verify that the completed behavior emits the cancel-tcpip-forward global request without requiring protocol knowledge from the caller.

Written by the indexing model from the issue text.