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/start-session.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+if [ -z "${TMUX}" ]; then
+    exec tmux new-session -s serial -d "$0"
+fi
+
+tmux \
+    set-option -t serial:0 -w remain-on-exit on \; \
+    rename-window -t serial:0 info
+
+cat <<'EOF'
+This window must remain open to keep the tmux server/session running.
+If all windows are closed, tmux will exit and will not be able to
+receive commands to add new windows for hotplugged devices.
+
+Use `respawn-window` to restart all serial terminal windows.
+EOF