getServersExt: send known info about dual-stack servers if requested

Clients request this information via the 'dual' option.
It is sent in an OOB packet, currently with the following structure:

It has the following format:
   0..3    OOB signature
   4..29   String "getServersExtResponseLinks"
  30       NUL
  31..end  Address pairs, consisting of:
           IPv4 address and port
           IPv6 address and port

Addresses and ports are sent in network byte order.

The packet is sent before getserversExtResponse.

This is experimental and may be changed.