add listen.send_recv_error config option (#670)

By default, Nebula replies to packets it has no tunnel for with a `recv_error` packet. This packet helps speed up re-connection
in the case that Nebula on either side did not shut down cleanly. This response can be abused as a way to discover if Nebula is running
on a host though. This option lets you configure if you want to send `recv_error` packets always, never, or only to private network remotes.
valid values: always, never, private

This setting is reloadable with SIGHUP.

config/config.go

@@ -76,6 +76,11 @@ func (c *C) RegisterReloadCallback(f func(*C)) {
 	c.callbacks = append(c.callbacks, f)
 }
 
+// InitialLoad returns true if this is the first load of the config, and ReloadConfig has not been called yet.
+func (c *C) InitialLoad() bool {
+	return c.oldSettings == nil
+}
+
 // HasChanged checks if the underlying structure of the provided key has changed after a config reload. The value of
 // k in both the old and new settings will be serialized, the result of the string comparison is returned.
 // If k is an empty string the entire config is tested.