stdio::flush() does not work

[lasiotus]

At the moment, stdio::flush() in moto-runtime src/stdio.rs is a NOOP, which leads to delayed (or lost?) output when a process terminates. The tricky part is that stdin/out/err are implemented as pipes, and bytes move through them via relay threads, so flushing means some kind of cross-process synchronization, as this is all happening in userspace. Or maybe it is not really difficult, and just adding flush() to sync_pipe will work?