Refactor poll_oneoff on *nix (#137)

* Fixes CraneStation/wasmtime#440

This commit introduces a couple of changes/fixes:
* it annotates `log::debug!` messages with "host" to differentiate
  between file descriptors stored on the WASI side (aka the wrappers)
  and those managed by the host (aka the wrapped)
* it fixes CraneStation/wasmtime#440, i.e., incorrect passing of
  file descriptor to `poll_oneoff` where currently errenously we
  pass in the wrapper instead of the wrapped value
* it adds a couple more `log::debug!` macros calls for easier future
  debugging

* Add partial refactorting to poll_oneoff

This commit lays the groundwork for more clean up to come in
subsequent commits.

* Finalise refactoring of `poll_oneoff`

* Fix compilation error on Windows

* Address majority of suggestions and refactor

Co-authored-by: Marcin Mielniczuk <marmistrz.dev@zoho.eu>

* Add poll_oneoff test case

* Leave timeout in nanoseconds in ClockEventData

Instead of converting the timeout value from nanoseconds to
milliseconds in the host-independent impl, move the conversion
to *nix-specific impl as the conversion is currently only warranted
by the POSIX `poll` syscall.

* Don't fail immediately on bad descriptor

If the user specifies an invalid descriptor inside a subscription,
don't fail immediately but rather generate an event with the thrown
WASI error code, and continue with the remaining, potentially
correct subscriptions.

src/ctx.rs

@@ -134,7 +134,9 @@ impl WasiCtxBuilder {
             }
             let mut fe = FdEntry::from(dir)?;
             fe.preopen_path = Some(guest_path);
+            log::debug!("WasiCtx inserting ({:?}, {:?})", preopen_fd, fe);
             self.fds.insert(preopen_fd, fe);
+            log::debug!("WasiCtx fds = {:?}", self.fds);
             preopen_fd = preopen_fd.checked_add(1).ok_or(Error::ENFILE)?;
         }