MPMs, core: make duplicated listeners (SO_REUSEPORT) introduced in r1599531

            less intrusive.

Submitted by: Yingqi Lu <yingqi.lu@intel.com>
Modified/Committed by: ylavic

Add ListenCoresBucketsRatio which is a configurable ratio between the number of
CPU cores (online) and the number of listeners buckets to create, defaulting to
zero (so that listeners buckets become an opt-in, ie. ncpus / ratio > 1).
This could also be made an opt-out by using the previous hardcoded value (8) as
default.

Make ap_close_listeners() act on all the listeners (including duplicated ones),
since the function is also called externally (eg. mod_cgid, mod_ssl_ct and
possibly any third party module) to cleanup opened descriptors when a process
is forked (the duplicated listeners are kept in a scoped/static variable).

Add ap_close_listeners_ex() to close a single bucket of listeners, used by the
children to close unused duplicates and internally by ap_close_listeners().

Make ap_duplicate_listeners() compute the number o...