add ipv6 support for xlxd-2.5.3

[jg1uaa]

This is reconstruction of PR #122, #162, #189 and #205.

BrandMeister, MMDVMHost and mrefd(M17) has already support IPv6.
Why xlxd not?

[LX1IQ]

Because the Icom Terminal mode is not compatible with IPv6

[jg1uaa]

Even if Icom Terminal mode is not avaliable, it is worth supporting IPv6 for others.

[fabianfrz]

@LX1IQ It would simplify a lot of setups because there is no NAT that needs to be considered in IPv6 as well as there is no private address room that needs to be considered as all hosts should have a global unicast address. This is important in times of CGN you might not get rid of.

That means:

If IPv6 is possible, use it
If not, fall back to IPv4 if possible (if the server has an IPv4 address)

The only thing is if there are reflectors, that are IPv6 or IPv4 only. They might not be reached but it is more an issue of an IPv6 only Server and IPv4 only client as in the other direction, there are mechanisms to cope with that issue (NAT64 etc.).

IPv6 has the following advantages:

• No timeouts in NAT tables that reset connections as they simply does not exist
• easier firewall rules in long term as you only need source and destination + ports - no need consider NAT
• IPv6 is more robust than IPv4 as it has a working path MTU discovery

Disadvantages:

• In the mean time, the reflectors probably need full dual stack and the firewall needs to be configured for both protocols
• Currently the ICOM software or ICOM radios do not support it but that is not an issue, it can still connect via IPv4 - there might be some reflectors, this software will not be able to connect to but that is ICOM's fault, not xlxd's. It is not pre 1996. Also it might not work anyway, as ther is no way back to your network due to carrier grade nat. Thanks to the providers for that.
• You likely have to adjust some kind of an reflector list. It needs to be ensured, that old xlxd versions are compatible with a newer version of the list that includes IPv6 addresses or it might break existing infrastructure and should be tested. For example: http://xlxapi.rlx.lu/api.php?do=GetReflectorList

[jg1uaa]

Well, how is the things going?
Still do nothing coming urfd (https://github.com/nostar/urfd)?
This supports IPv6 and "URF is superset of XLX".