Correct terminal type handling

[russjones]

Problem

Teleport tries to read in the terminal type from environment variables on the server, this should be done on the client and the server should request the terminal type passed in pty-req.

Proposed Solution

On the server side, when teleport processing pty-req extract the terminal type and store it in the context and use that when requesting a terminal.

On the client side when tsh is sending requests, it should extract TERM from the environment of the user and pack this into a pty-req. If TERM is not set, then use xterm-256color (not sure about this last bit yet).

[russjones]

Fixed in Teleport 2.4.0.