Revive support for terminal multiplexers

[maxonthegit]

At present, vstart offers low-level support for attaching consoles to pseudo-terminals. The user then has to manually attach a terminal multiplexer (e.g., screen, tmux) to these terminals in order to get the actual consoles.
This process could be made automatic, i.e., in such a way that a user can tell vstart to attach a console directly to a terminal multiplexer.

This could allow users of text-only environments to play nicely with Netkit.