Have (*config.C).CatchHUP() return early when there is no file
path available from which to reload.
This will allow wrapping service to manage their own signal
trapping (which is particularly important if they've used
config from a string.
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.
Dave Russell