BUG/MINOR: sock_inet: use SO_REUSEPORT_LB where available

BUG/MINOR: sock_inet: use SO_REUSEPORT_LB where available

On FreeBSD 13.1 I noticed that thread balancing using shards was not
always working. Sometimes several threads would work, but most of the
time a single one was taking all the traffic. This is related to how
SO_REUSEPORT works on FreeBSD since version 12, as it seems there is
no guarantee that multiple sockets will receive the traffic. However
there is SO_REUSEPORT_LB that is designed exactly for this, so we'd
rather use it when available.

This patch may possibly be backported, but nobody complained and it's
not sure that many users rely on shards. So better wait for some feedback
before backporting this.