MINOR: listener: automatically adjust shards based on support for SO_REUSEPORT

MINOR: listener: automatically adjust shards based on support for SO_REUSEPORT

Now if multiple shards are explicitly requested, and the listener's
protocol doesn't support SO_REUSEPORT, sharding is disabled, which will
result in the socket being automatically duped if needed. A warning is
emitted when this happens. If "shards by-group" or "shards by-thread"
are used, these will automatically be turned down to 1 since we want
this to be possible easily using -dR on the command line without having
to djust the config. For "by-thread", a diag warning will be emitted to
help troubleshoot possible performance issues.