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-window@.service

@@ -0,0 +1,16 @@
+# vim: set ft=systemd :
+[Unit]
+Description=Add serial terminal window for %I
+Requires=serial-terminal-server.service
+After=serial-terminal-server.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/libexec/serterm/add-window %I
+ExecStop=/usr/local/libexec/serterm/remove-window %I
+RemainAfterExit=yes
+User=serterm
+Group=serterm
+
+[Install]
+WantedBy=serial-terminal-server.service