Mirrors the agent→forwarder pattern: `decnet swarmctl` now fires the
syslog-TLS listener as a detached Popen sibling so a single master
invocation brings the full receive pipeline online. --no-listener opts
out for operators who want to run the listener on a different host (or
under their own systemd unit).
Listener bind host / port come from DECNET_LISTENER_HOST and
DECNET_SWARM_SYSLOG_PORT — both seedable from /etc/decnet/decnet.ini.
PID at $(pid_dir)/listener.pid so operators can kill/restart manually.
decnet.ini.example ships alongside env.config.example as the
documented surface for the new role-scoped config. Mode, forwarder
targets, listener bind, and master ports all live there — no more
memorizing flag trees.
Extends tests/test_auto_spawn.py with two swarmctl cases: listener is
spawned with the expected argv + PID file, and --no-listener suppresses.
New _spawn_detached(argv, pid_file) helper uses Popen with
start_new_session=True + close_fds=True + DEVNULL stdio to launch a
DECNET subcommand as a fully independent process. The parent does NOT
wait(); if it dies the child survives under init. This is deliberately
not a supervisor — if the child dies the operator restarts it manually.
_pid_dir() picks /opt/decnet when writable else ~/.decnet, so both
root-run production and non-root dev work without ceremony.
`decnet agent` now auto-spawns `decnet forwarder --daemon ...` as
that detached sibling, pulling master host + syslog port from
DECNET_SWARM_MASTER_HOST / DECNET_SWARM_SYSLOG_PORT. --no-forwarder
opts out. If DECNET_SWARM_MASTER_HOST is unset the auto-spawn is
silently skipped (single-host dev or operator wants to start the
forwarder separately).
tests/test_auto_spawn.py monkeypatches subprocess.Popen and verifies:
the detach kwargs are passed, the PID file exists and contains a
valid positive integer (PID-file corruption is a real operational
headache — catching bad writes at the test level is free), the
--no-forwarder flag suppresses the spawn, and the unset-master-host
path silently skips.
