Reduce CPU load due to unnecessary accept() calls

Each time a listener socket becomes ready, MariaDB calls accept() ten times (MAX_ACCEPT_RETRY), even if all but the first one return EAGAIN because there are no more connections.
This causes unnecessary CPU usage - on our server, the CPU load of that thread, which does nothing but accept(), saturates one CPU core by ~45%.
The loop should stop after the first EAGAIN.