MDEV-34994: sql/mysqld: stop accept() loop after the first EAGAIN

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.

Perf report:

    11.01%  mariadbd  libc.so.6          [.] accept4
     6.42%  mariadbd  [kernel.kallsyms]  [k] finish_task_switch.isra.0
     5.50%  mariadbd  [kernel.kallsyms]  [k] _raw_spin_unlock_irqrestore
     5.50%  mariadbd  [kernel.kallsyms]  [k] syscall_enter_from_user_mode
     4.59%  mariadbd  [kernel.kallsyms]  [k] __fget_light
     3.67%  mariadbd  [kernel.kallsyms]  [k] kmem_cache_alloc
     2.75%  mariadbd  [kernel.kallsyms]  [k] fput
     2.75%  mariadbd  [kernel.kallsyms]  [k] mod_objcg_state
     1.83%  mariadbd  [kernel.kallsyms]  [k] __inode_wait_for_writeback
     1.83%  mariadbd  [kernel.kallsyms]  [k] __sys_accept4
     1.83%  mariadbd  [kernel.kallsyms]  [k] _raw_spin_unlock_irq
     1.83%  mariadbd  [kernel.kallsyms]  [k] alloc_inode
     1.83%  mariadbd  [kernel.kallsyms]  [k] call_rcu

sql/mysqld.cc

@@ -6360,7 +6360,9 @@ void handle_connections_sockets()
                                     &length);
       if (mysql_socket_getfd(new_sock) != INVALID_SOCKET)
         handle_accepted_socket(new_sock, sock);
-      else if (socket_errno != SOCKET_EINTR && socket_errno != SOCKET_EAGAIN)
+      else if (socket_errno == SOCKET_EAGAIN || socket_errno == SOCKET_EWOULDBLOCK)
+        break;
+      else if (socket_errno != SOCKET_EINTR)
       {
         /*
           accept(2) failed on the listening port.