Note: `GNU/screen` has a different interpretation of what constitutes
a "vertical split", hence the inverted command descriptions, compared
to the tmux/iterm etc.
Closes#1626
- The proper way to do this would be to switch focus to the target client's region but GNU Screen offers no obvious way to do that.
- Remove screen region after client is closed.
- Assumed: Kak server runs in screen.
- Hack: kak sets `/proc/self/fd/0 -> /dev/null`. Get the client process tty because Screen needs to know the controlling terminal. Else Screen will use the last known tty and will open new windows on a different terminal if one is connected.
