poll: add stdout fflush after poll query

poll: add stdout fflush after poll query

ATM it's a bit ugly to enforce flushing of 'stdio' here, but works as quick
hot-fix.

log_print*() is using buffered I/O.

But for pooling with typical 1s interval this may take a while before
buffer about continues progress gets flushed.
So ATM fflush().

TODO: either add  log_print*_with_flush() or maybe directly use just
line buffering with log_print() and only log_debug() keep using buffered
I/O mode.