r/serterm: Deploy serial terminal multiplexer

Using `tmux`, we can spawn a bunch of `picocom` processes for the serial
ports connected to other server's console ports.  The
_serial-terminal-server_ service manages the `tmux` server process,
while the individual _serial-terminal-server-window@.service_ units
create a window in the `tmux` session.

The serial terminal server runs as a dedicated user.  The SSH server is
configured to force this user to connect to the `tmux` session.  This
should help ensure the serial consoles are accessible, even if the
Active Directory server is unavailable.

roles/serterm/files/remove-window.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+port="$1"
+
+exec tmux kill-window -t "${port##*/}"