For threaded resolving, added an additional socket/eventfd pair to the
multi handle for notifications from threads. The original "double use"
of the standard wakeup pair did lead to regressions for apps.
The API definition of curl_multi_poll/wait/wakeup is pretty tight
regarding what effects what and adding notifications on top of that
broke what apps perceived to be the contract.
Fixes#22272
Reported-by: Sergei Zimmerman
Closes#22274
