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_iface.go

@@ -0,0 +1,14 @@
+package flock
+
+// documentation for github.com/gofrs/flock incorrectly claims that
+// Flock implements sync.Locker; it does not because the Unlock method
+// has a return type (err).
+type Flocker interface {
+	Unlock() error
+}
+
+type noopFlocker struct{}
+
+func (n *noopFlocker) Unlock() error {
+	return nil
+}