fix(console): disable the unauthenticated serial console by default

socketiod does not validate the console token, keeps a single global PTY
and broadcasts its output to every connected client, so anyone who can
reach port 6081 can read and write a serial console that someone opened.
It is also broken for legitimate users, since console/views.py now writes
signed tokens that socketiod cannot parse.

Until it is rewritten, turn the serial console off by default:

- Add SERIAL_CONSOLE_ENABLED = False to the settings template; installs
  whose settings.py lacks it are treated as disabled.
- The console view shows an error instead of the xterm page for pty
  consoles while the setting is off.
- socketiod refuses to start unless SERIAL_CONSOLE_ENABLED is True.
- Supervisor: autostart=false for socketiod; drop -d (debug) from the
  supervisor and systemd definitions; fix the systemd unit description;
  conf/systemd/install.sh no longer suggests enabling socketiod.
- README: stop opening port 6081 and document how to enable the serial
  console on a trusted network.

console/socketiod

@@ -17,6 +17,19 @@ if ROOT_PATH not in sys.path:
 
 django.setup()
 
+from django.conf import settings as django_settings
+
+# socketiod does not authenticate connections yet (ROADMAP S-01): any client
+# that can reach the port can read and write an open serial console. Refuse
+# to start unless an admin explicitly opts in.
+if not getattr(django_settings, "SERIAL_CONSOLE_ENABLED", False):
+    sys.stderr.write(
+        "socketiod: the serial console is disabled (SERIAL_CONSOLE_ENABLED is not "
+        "True in settings). It is unauthenticated; only enable it on a trusted "
+        "network.\n"
+    )
+    sys.exit(1)
+
 import re
 import tempfile
 import io