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/serial-terminal-server.service

@@ -0,0 +1,14 @@
+# vim: set ft=systemd :
+[Unit]
+Description=Serial Terminal
+
+[Service]
+Type=forking
+ExecStart=/usr/local/libexec/serterm/start-server
+User=serterm
+Group=serterm
+Restart=always
+RestartSec=1s
+
+[Install]
+WantedBy=multi-user.target