fix: Drain shutdown event

[ankush]

Otherwise it goes in a spinny loop. The flag numbers were also wrong, as per https://man7.org/linux/man-pages/man2/eventfd.2.html

The file descriptor is readable (the select(2) readfds argument; the poll(2) POLLIN flag) if the counter has a value greater than 0.

AND

If EFD_SEMAPHORE was not specified and the eventfd counter has a nonzero value, then a read(2) returns 8 bytes containing that value, and the counter's value is reset to zero.