core: Disallow multiple Listen on the same IP:port when listener buckets
are configured (ListenCoresBucketsRatio > 0), consistently with the single
bucket case (default), thus fixing the leak of the corresponding socket
descriptors on graceful restart.



Follow up to r1789220.
Document the implicit behaviour of ListenCoresBucketsRatio when multiple
Listen-ers are configured on the same IP:port.


Submitted by: ylavic
Reviewed by: ylavic, jim, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1805221 13f79535-47bb-0310-9956-ffa450edef68