Use a worker thread for stdin on Unix with preview2

Currently stdin on Unix uses `AsyncFd` after setting the nonblocking flag
on the stdin file descriptor. This cannot generally be relied on to
succeed or work well in cases such as:

* On Linux if stdin is a file (e.g. `wasmtime < ./foo.txt`) then that
  can't be added to epoll and `epoll_ctl` fails with EPERM.

* On macOS if stdin is `/dev/null` that can't be added to kqueue and
  fails with `EINVAL`.

Currently this causes Wasmtime to panic due to a `.unwrap()` in failing
to register stdin. To fix this this commit removes the Unix-specific
logic using `AsyncFd` and instead falls back to unconditionally using a
spawned thread like Windows does. This does mean it's a little less
lightweight but should be more robust in situations like the above.

crates/wasi/src/preview2/stdio.rs

@@ -1,13 +1,6 @@
 use crate::preview2::pipe::AsyncWriteStream;
 
-#[cfg(unix)]
-mod unix;
-#[cfg(unix)]
-pub use self::unix::{stdin, Stdin};
-
-#[allow(dead_code)]
 mod worker_thread_stdin;
-#[cfg(windows)]
 pub use self::worker_thread_stdin::{stdin, Stdin};
 
 pub type Stdout = AsyncWriteStream;