rec: Use one listening socket per thread when reuseport is enabled

rec: Use one listening socket per thread when reuseport is enabled

Except if `pdns-distributes-queries` is true, of course.
We used to shared the same listening socket between all threads
when `pdns-distributes-queries` is set to false, even with `reuseport`
set to true and `SO_REUSEPORT` support available. After this commit:
* if `pdns-distributes-queries` is true, the distributor thread
is still the only one listening to incoming queries
* if `pdns-distributes-queries` is false and either `reuseport` is
false or `SO_REUSEPORT` support is not available, all threads share
the same listening socket as it was before
* if `pdns-distributes-queries` is false, `SO_REUSEPORT` support is
available and `reuseport` is true, we open a separate listening
socket per thread to let the kernel distribute the incoming queries
for us, avoiding any thundering herd issue as well as the distributor
thread being a bottleneck.