osmo_io: add osmo_iofd_flush() to transmit the pending Tx queue
osmo_iofd_write_msgb() and friends only enqueue the given message; the actual transmission happens asynchronously from within osmo_select_main(). An application that needs to say 'goodbye' right before calling osmo_iofd_free() (think of a connection teardown datagram, like TRXC "CMD POWEROFF") currently has no way to do so reliably: osmo_iofd_free() discards the Tx queue, so a message enqueued in the same event loop iteration never reaches the socket.
Add osmo_iofd_flush(), which synchronously transmits the pending Tx queue, stopping on -EAGAIN or other errors. Only the poll backend implements it for now; the io_uring backend returns -ENOTSUP, since in-flight SQE submissions make a synchronous flush non-trivial there.
