Add support for FreeBSD's SO_REUSEPORT_LB

[rgacogne]

Short description

On FreeBSD, SO_REUSEPORT "permits multiple instances of a program to each receive UDP/IP multicast or broadcast datagrams destined for the bound port", while SO_REUSEPORT_LB distributes them "among the sharing processes based on a hash function of local port number, foreign IP address and port number".

I tested that SO_REUSEPORT is still correctly enabled on Linux after this change, but not that SO_REUSEPORT_LB is actually enabled on FreeBSD.

Closes #9156.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[omoerbeek]

It would be good to have a confirmation SO_REUSEPORT_LB is indeed used on FreeBSD

[rgacogne]

It would be good to have a confirmation SO_REUSEPORT_LB is indeed used on FreeBSD

Agreed, let's try to get some FreeBSD user interested in testing this :-)

[kbowlingns]

We left SO_REUSEPORT intact on FreeBSD because it allows staging a software upgrade for instance, where you gradually drain connections from the first image and the second starts to take the new ones. Not sure if it is worth exposing both options to FreeBSD users, most would probably prefer the _LB variant.

[Habbie]

@RvdE reports seeing _LB in a trace. He also reports getting duplicate TCP sockets but not duplicate UDP sockets.

[RalfvdEnden]

Just double checked and it works for udp(4|6) as well. Must have been looking at the TLS port (853) earlier, which I don't set the 'reusePort=true' on (not even sure it's supported).

[rgacogne]

Thanks for checking! Habbie, Otto, any objection to merging this?

[Habbie]

do it