- Write tests.

relay-ctrl-age:

relay-ctrl-allow:

relay-ctrl-check:

relay-ctrl-send:

- Modify the protocol to send $USER and $DOMAIN.

- Modify the protocol to send a timestamp and a cryptographic signature
  of all the rest of the packet + a secret.

relay-ctrl-udp:

- Save the sent environment variables into the file.

- Restrict the set of allowed senders, using directory entries to
  enumerate the IPs.

- Check the signatures of incoming packets.  Store signatures for
  seperate servers seperately.

- Bind the socket to a specific IP instead of IPV4ADDR_ANY.
