Removed FD_CLOEXEC setting: already done by mysql_socket_accept().

Moved O_NONBLOCK setting out of accepting loop.

Removed blocking acceptance attempt, which was dead code. It was supposed
to be executed before the last iteration, however it was actually executed
during the last iteration. And it is not correct to block on one socket
leaving other sockets unattended anyway.

Gives ~5% throughput improvemet in sysbench connect benchmark.

Part of MDEV-19515 - Improve connect speed