optionally protect against multiple starts with flock (#1873)

* optionally protect against multiple starts with flock

Fixes #1823

* use traditional .lock extension

* move config key to top level

irc/flock/flock_plan9.go

@@ -0,0 +1,7 @@
+//go:build plan9
+
+package flock
+
+func TryAcquireFlock(path string) (fl Flocker, err error) {
+	return &noopFlocker{}, nil
+}